Allow returning 0 values in ft:facets#3 and ft:facets#4 functions (for consistency with similar functions)

[daliboris]

Is your feature request related to a problem? Please describe.
The proposed feature will ensure consistency when calling functions to return range, field and facet values for node set.

The range:index-keys-for-field#3, range:index-keys-for-field#4, util:index-keys#4, util:index-keys#5, and ft:index-keys-for-field#4 functions accept $max-number-returned as xs:int? parameter defined as The maximum number of returned keys (or The maximum number of keys to return).

The ft:facets#3 and ft:facets#4 functions accept $count as xs:integer? parameter defined as The number of facet labels to be returned.

While range, util and ft index keys functions return an empty set of items when $max-number-returned parameter is set to 0, ft:facet functions throw topN must be > 0 (got: 0) error with following trace.

at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:618) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:127) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:235) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:1624) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:233) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1440) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:188) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:505) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:1594) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:186) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1355) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.Dispatcher.forward(Dispatcher.java:167) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.Dispatcher.forward(Dispatcher.java:81) ~[exist.uber.jar:6.4.0]
at org.exist.http.urlrewrite.Forward.doRewrite(Forward.java:50) ~[exist.uber.jar:6.4.0]
at org.exist.http.urlrewrite.XQueryURLRewrite.doRewrite(XQueryURLRewrite.java:523) ~[exist.uber.jar:6.4.0]
at org.exist.http.urlrewrite.XQueryURLRewrite.service(XQueryURLRewrite.java:340) ~[exist.uber.jar:6.4.0]
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.servlet.ServletHolder$NotAsync.service(ServletHolder.java:1450) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.servlet.ServletHolder.handle(ServletHolder.java:799) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.servlet.ServletHandler$ChainEnd.doFilter(ServletHandler.java:1656) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.websocket.server.WebSocketUpgradeFilter.doFilter(WebSocketUpgradeFilter.java:292) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.servlet.FilterHolder.doFilter(FilterHolder.java:193) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.servlet.ServletHandler$Chain.doFilter(ServletHandler.java:1626) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.servlet.ServletHandler.doHandle(ServletHandler.java:552) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:143) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.security.SecurityHandler.handle(SecurityHandler.java:571) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:127) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:235) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.session.SessionHandler.doHandle(SessionHandler.java:1624) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.ScopedHandler.nextHandle(ScopedHandler.java:233) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.ContextHandler.doHandle(ContextHandler.java:1440) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:188) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.servlet.ServletHandler.doScope(ServletHandler.java:505) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.session.SessionHandler.doScope(SessionHandler.java:1594) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.ScopedHandler.nextScope(ScopedHandler.java:186) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.ContextHandler.doScope(ContextHandler.java:1355) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.ScopedHandler.handle(ScopedHandler.java:141) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:234) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.HandlerCollection.handle(HandlerCollection.java:146) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.gzip.GzipHandler.handle(GzipHandler.java:722) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.handler.HandlerWrapper.handle(HandlerWrapper.java:127) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.Server.handle(Server.java:516) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.HttpChannel.lambda$handle$1(HttpChannel.java:487) ~[exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.HttpChannel.dispatch(HttpChannel.java:732) [exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.HttpChannel.handle(HttpChannel.java:479) [exist.uber.jar:6.4.0]
at org.eclipse.jetty.server.HttpConnection.onFillable(HttpConnection.java:277) [exist.uber.jar:6.4.0]
at org.eclipse.jetty.io.AbstractConnection$ReadCallback.succeeded(AbstractConnection.java:311) [exist.uber.jar:6.4.0]
at org.eclipse.jetty.io.FillInterest.fillable(FillInterest.java:105) [exist.uber.jar:6.4.0]
at org.eclipse.jetty.io.ChannelEndPoint$1.run(ChannelEndPoint.java:104) [exist.uber.jar:6.4.0]
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.runTask(EatWhatYouKill.java:338) [exist.uber.jar:6.4.0]
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.doProduce(EatWhatYouKill.java:315) [exist.uber.jar:6.4.0]
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.tryProduce(EatWhatYouKill.java:173) [exist.uber.jar:6.4.0]
at org.eclipse.jetty.util.thread.strategy.EatWhatYouKill.run(EatWhatYouKill.java:131) [exist.uber.jar:6.4.0]
at org.eclipse.jetty.util.thread.ReservedThreadExecutor$ReservedThread.run(ReservedThreadExecutor.java:409) [exist.uber.jar:6.4.0]
at org.eclipse.jetty.util.thread.QueuedThreadPool.runJob(QueuedThreadPool.java:883) [exist.uber.jar:6.4.0]
at org.eclipse.jetty.util.thread.QueuedThreadPool$Runner.run(QueuedThreadPool.java:1034) [exist.uber.jar:6.4.0]
at java.lang.Thread.run(Thread.java:750) [?:1.8.0_452]

Describe the solution you'd like
Allow $max-number-returned parameter to be 0 for ft:facets#3 and ft:facets#4 functions. If the parameter is equal to zero, return empty set of values.

Additional context
The question is, if returning 0 keys/values makes sense, but this issue is about the consistency of functions with similar purpose.