Move `compare-to-empty-string` to nursery (#5264)
## Summary
This rule has too many false positives. It has parity with the Pylint
version, but the Pylint version is part of an
[extension](https://pylint.readthedocs.io/en/stable/user_guide/messages/convention/compare-to-empty-string.html),
and so requires explicit opt-in.
I'm moving this rule to the nursery to require explicit opt-in, as with
Pylint.
Closes #4282.
