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 #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.

[sphuber]

Closing in favor of #6552

[giovannipizzi]

Apologies for creating a new issue, I looked for this but couldn't find it somehow!