Fix CollectionUtils collate keeping duplicate nulls when includeDuplicates is false

[rootvector2]

collate used lastItem == null as its first-iteration sentinel, so a real null element (legal when a null-tolerant Comparator like Comparator.nullsFirst is supplied) was treated as unseen and re-added, leaving duplicate nulls in the result when includeDuplicates is false; found while tracing the merge dedup path with null-permitting comparators.

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[garydgregory]

Good one @rootvector2 , merged 🚀

[garydgregory]

In the future, don't edit changes.xml.

[rootvector2]

thanks for merging, and noted on changes.xml, I'll leave it out of future PRs.