Add 'Is empty (NULL)' and 'Is not empty (NOT NULL)' operators to boolean field filtering

Problem/Motivation

Issue split from #2769407: Views UI should offer a 'Treat NULL values as FALSE' on boolean field filtering to expose the already existing code path for handling this for a clearer issue scope.

Proposed resolution

Views provides an option for filtering boolean fields on 'Is empty (NULL)' and 'Is not empty (NOT NULL)'

Remaining tasks

1. Write a patch
2. Review
3. Commit

User interface changes

Views now offers 'Is empty (NULL)' and 'Is not empty (NOT NULL)' operators to boolean field filtering

API changes

None.

Data model changes

None.

Release notes snippet

Views now offers 'Is empty (NULL)' and 'Is not empty (NOT NULL)' operators to boolean field filtering