[MRG] handle empty TSVs robustly and warn

[sappelhoff]

PR Description

closes #925

Merge checklist

Maintainer, please confirm the following before merging.
If applicable:

• All comments are resolved
• This is not your own PR
• All CIs are happy
• PR title starts with [MRG]
• whats_new.rst is updated
• New contributors have been added to CITATION.cff
• PR description includes phrase "closes <#issue-number>"

[codecov]

Codecov Report

Merging #1038 (fd853ef) into main (be6bd62) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1038      +/-   ##
==========================================
- Coverage   95.16%   95.15%   -0.02%     
==========================================
  Files          25       25              
  Lines        3786     3796      +10     
==========================================
+ Hits         3603     3612       +9     
- Misses        183      184       +1     

Impacted Files	Coverage Δ
mne_bids/tsv_handler.py	100.00% <100.00%> (ø)
mne_bids/write.py	97.04% <0.00%> (-0.11%)	⬇️

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[hoechenberger]

Nice, just one small request

[hoechenberger]

LGTM!