QueryBuilder: implement the `has_key` operator properly for SQLite based storage plugins

[sphuber]

This was originally implemented, but the implementation (silently) yielded incorrect results, so it was removed out of precaution in https://github.com/aiidateam/aiida-core/pull/6497 When implementing it, special attention should be paid to it functioning properly in combination with the negation operator and in combination with other filter operations.