NIFI-14043 Add support for keySet in HazelcastMapCacheClient

[EndzeitBegins]

Summary

NIFI-14043

Applies minor refactoring (e.g. x.isEmpty() instead of !x.isPresent()).
Aligns names of private (de)serialisation methods.

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  • JDK 21

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[exceptionfactory]

Thanks for the improvement @EndzeitBegins, this addition looks good. +1 merging