Remove `NullLocker` and satisfy `mypy` 0.990

[moonsikpark]

Mypy introduced python/mypy#12118, and it wails empty-body on functions with empty body like this.

poetry/src/poetry/packages/locker.py

Lines 469 to 471 in b28339d

	class NullLocker(Locker):
	def set_lock_data(self, root: Package, packages: list[Package]) -> bool:
	pass

It seems there are no references to NullLocker, so I am wondering whether we can remove NullLocker and bump the mypy version to 0.990. If that's OK, I'm ready to submit a PR.

[abn]

This was originally used in the update command, which is not the case anymore.

Based on a quick scan of https://cs.github.com/?q=NullLocker+path%3A*.py it seems there is no downstream usage on GitHub that we are aware of that will break implicitly.

So, I suspect we can drop this without any side-effects. I doubt there is much value in employing a deprecation cycle here.

We should call it out in the change log though. There are tools out there that use poetry as a library.

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