MRG: Fix handling of n/a event values when auto-renaming events on read

[hoechenberger]

Fixes #936

Old:

The event "left" refers to multiple event values. Creating hierarchical event names.
    Renaming event: left -> left/n/a
    Renaming event: left -> left/n/a
    Renaming event: left -> left/1
    Renaming event: left -> left/n/a
    Renaming event: left -> left/n/a

New:

The event "left" refers to multiple event values. Creating hierarchical event names.
    Renaming event: left -> left/na
    Renaming event: left -> left/na
    Renaming event: left -> left/1
    Renaming event: left -> left/na
    Renaming event: left -> left/na

WIP:

• Add tests
• Add changelog entry

Merge checklist

Maintainer, please confirm the following before merging:

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

[hoechenberger]

CI failure is on Python 3.10 is unrelated to these changes

[agramfort]

don't know if CI issue is related

[hoechenberger]

@agramfort CI issues seem to be fixed via 9fa710e, let's hope all comes back green now!

[codecov]

Codecov Report

Merging #937 (33f63c9) into main (a497390) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #937   +/-   ##
=======================================
  Coverage   94.78%   94.78%           
=======================================
  Files          23       23           
  Lines        3490     3492    +2     
=======================================
+ Hits         3308     3310    +2     
  Misses        182      182           

Impacted Files	Coverage Δ
mne_bids/read.py	95.84% <100.00%> (+0.02%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a497390...33f63c9. Read the comment docs.