feat(query): add isnull operator for NULL checks

[joshdimanteto]

This PR will close #527

Description

see #527

Testing Instructions

Add a set up instructions describing how the reviewer should test the code

• Review code
• Check GitHub Actions build
• If icatdb Generator Script Consistency Test CI job fails, is this because of a deliberate change made to the script to change generated data (which isn't actually a problem) or is here an underlying issue with the changes made?
• Review changes to test coverage
• Does this change mean a new patch, minor or major version should be made? If so, does one of the commit messages feature fix:, feat: or BREAKING CHANGE: so a release is automatically made via GitHub Actions upon merge?
• {more steps here}

Agile Board Tracking

#527

[patrick-austin]

Code looks OK to me but haven't had a chance to run it yet, will try and do that when I can before approving.

I note that this is not the solution that @louise-davies suggested - are you happy enough with this as is and dealing with a general solution for the discrepancy between boolean equality and is-ness (not sure what the word for that is - identity?) when/if it comes to it?

[louise-davies]

Code looks OK to me but haven't had a chance to run it yet, will try and do that when I can before approving.

I note that this is not the solution that @louise-davies suggested - are you happy enough with this as is and dealing with a general solution for the discrepancy between boolean equality and is-ness (not sure what the word for that is - identity?) when/if it comes to it?

@patrick-austin Apparently JPQL doesn't support the IS operator, just specific IS NULL (and also IS EMPTY) so it had to be the specific isnull operator. What I was looking up was SQL specific 😓

[patrick-austin]

Tried it on my dev ICAT and gave the expected number of results for "isnull": true and "isnull": false.