HSQLDB support for Multiset

[retroandchill]

Currently if you try to use a MULTISET fetch strategy against and HSQL database it causes an error with a NullPointerException, caused by there not being any support for it for the provider. I partially understand that this may have to do with the fact that HSQLDB lacks JSON support, however I only figured out what was wrong by digging into the source code.

What should probably be done is the following:

1. Provide a better more direct error message when a multiset select fails
2. Allow for a fallback for cases where there are multiple database providers in use (make it configurable if possible)
3. Investigate if there is any way to enable support for HSQLDB for multiset

[beikov]

Hi and thanks for the feedback. Actually, HSQLDB has JSON support as of version 2.7, but Blaze-Persistence neither tests with, nor has extended support for HSQLDB. That database simply was never of interest to users so far. We only support/test with H2.

Support for HSQLDB might be something that we could consider for Blaze-Persistence 2.0 though. Since that version will drop support for old Hibernate ORM versions it will be a lot easier to enable testing and doing verification.