Persistence store commit returns null fix, passing test added. Simulates pipeline pre-commit hook returning a false.
Merge pull request #368 from hudo/pipelinehook-issue

Persistence store commit returns null fix, passing test added.
Merge remote-tracking branch 'main/master' into observable-cleanup

Conflicts:
	src/NEventStore.Tests/OptimisticEventStreamTests.cs
	src/NEventStore/OptimisticEventStream.cs