[MRG] BUG: Fix dtype bug for counts

[larsoner]

PR Description

Counts should be integers, make them that way by filling with -1 then converting to pandas Int64Dtype with pd.NA.

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

All modified and coverable lines are covered by tests ✅

Comparison is base (87eea28) 97.61% compared to head (8f2655a) 97.61%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1227   +/-   ##
=======================================
  Coverage   97.61%   97.61%           
=======================================
  Files          40       40           
  Lines        8685     8686    +1     
=======================================
+ Hits         8478     8479    +1     
  Misses        207      207           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[agramfort]

thx @larsoner