[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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

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

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[agramfort]

thx @larsoner