BUG: Fix bug with logging level

[larsoner]

In MNE-BIDS-Pipeline in the find_bads_maxwell step I was getting a ton of messages about windowing even though verbose=False was being used.

I tracked it down to a weird case where _verbose_safe_false() returned None because logger.level was already high enough not to log... but use_log_level(None) did not mean "don't modify the logger level", it meant "read the sys config and use the default" 🤦 use_log_level is used internally by the verbose decorator, so making it be a no-op when verbose is None makes the most sense here. It also fixes the behavior of set_log_level, which previously probably didn't do what we wanted it to.

With this PR, we should have a lot fewer accesses to the .mne/mne-python.json file as well, which is nice I think (similar to #13777).