Allow prune to remove nil values

[nellicus]

allow to easily and dynamically remove nil values

e.g. remove all fields containing nil

prune{ blacklist_values => [ "\S+", nil] }

[talevy]

Closing in favor of tracking this in the plugin repo

re: logstash-plugins/logstash-filter-prune#10

[magnusbaeck]

Closing in favor of tracking this in the plugin repo

Okay, but note that that issue covers nil field names but this issue is for filtering field values.