HHH-18217 Properly handle empty value bindings for merge operations

[beikov]

[Please describe here what your change is about]

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https://hibernate.atlassian.net/browse/HHH-19725
https://hibernate.atlassian.net/browse/HHH-18217

[gavinking]

Thanks for working on this one, Christian.

One doubt I have is: there are two different cases here:

• management of secondary table updates (the original purpose of this code)
• upserts of primary tables (my abuse of Steve's original code)

Looking at these changes I can't clearly see where the difference between the two usages is manifest. It might be enough to just add a comment or something, but I would definitely like maintainers to have some warning about this subtlety. WDYT?

[beikov]

OptionalTableUpdateOperation and DeleteOrUpsertOperation both implement insert-update-or-delete semantics and the information to decide on the action comes from the passed ValuesAnalysis#getTablesWithNonNullValues().

When a table has non-null values, it will upsert, otherwise it will be deleted. To force upsert semantics, MergeCoordinator#analyzeUpdateValues makes sure to adapt the ValuesAnalysis to avoid deleting rows from optional tables when it would end up with just null values and instead always do an upsert.

I'm not sure where the best place for such a comment would be, but I added a summary of what I described here already to MergeCoordinator#analyzeUpdateValues. Let me know if you think that we need more comments and where you think they'd be appropriate.