Backport PR #1911 on branch 0.11.x (fix outer concatenation along var when varm is not empty)

[meeseeksmachine]

Backport PR #1911: fix outer concatenation along var when varm is not empty

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.72%. Comparing base (632dd95) to head (8b69bc9).
Report is 13 commits behind head on 0.11.x.

Additional details and impacted files

@@            Coverage Diff             @@
##           0.11.x    #1936      +/-   ##
==========================================
- Coverage   85.99%   83.72%   -2.27%     
==========================================
  Files          41       41              
  Lines        6277     6281       +4     
==========================================
- Hits         5398     5259     -139     
- Misses        879     1022     +143     

Files with missing lines	Coverage Δ
src/anndata/_core/merge.py	83.11% <100.00%> (-10.73%)	⬇️

... and 6 files with indirect coverage changes

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