Check for empty skip_file_prefixes in B028

[dylwil3]

As of #503 (cf #497) the following emits no lint:

❯ cat example.py
import warnings

warnings.warn("test", skip_file_prefixes=())

❯ uvx --with git+https://github.com/PyCQA/flake8-bugbear flake8 --select B028 check example.py

❯ echo $?
0

But it probably should: the stacklevel is only set to at least 2 if there is at least one prefix given. Of course, the user may pass in a reference to some unknown tuple, in which case it's probably better to skip the lint - but if they explicitly pass an empty tuple, then it makes sense to still emit the lint.

On the other hand I understand if this is overly pedantic and unlikely, so feel free to close the issue as unplanned!

[JelleZijlstra]

I'd accept a PR but this feels unlikely to matter in practice.