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]

Suggested change

	if ( cve.getKind() == UNIQUE ) {
	if ( valueBindings.isEmpty() ) {
	// Ignore primary key violation if the insert is composed of just the primary key
	return;
	}
	// assume it was the primary key constraint which was violated,
	// due to a new version of the row existing in the database
	throw new StaleStateException( mutationTarget.getRolePath(), cve );
	}
	throw cve;
	if ( cve.getKind() == UNIQUE ) {
	// Ignore primary key violation if the insert is composed of just the primary key
	if ( !valueBindings.isEmpty() ) {
	// assume it was the primary key constraint which was violated,
	// due to a new version of the row existing in the database
	throw new StaleStateException( mutationTarget.getRolePath(), cve );
	}
	}
	else {
	throw cve;
	}

[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?