Fix BIDSPath.basename for cases where only an extension was provided

[hoechenberger]

#%%
from mne_bids import BIDSPath
bp = BIDSPath(extension='.vhdr')
bp

main:

this PR:

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• [ ] All comments are resolved
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[hoechenberger]

I believe there was still something missing, I don't remember. I can try to take a look within the next couple of days