LANG-1813: Fix ArrayUtils.hashCode Javadoc for null input

[inponomarev]

Fix the Javadoc for ArrayUtils#hashCode(...) to remove the incorrect promise that a null input “returns zero”, while still documenting that null is a valid argument (https://issues.apache.org/jira/browse/LANG-1813)

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[garydgregory]

Just so you know, I made a comment in LANG-1813.