[FIX] ensure that dataframes can be used as input for second level GLM (estimation and contrast)

[Remi-Gau]

• Closes [BUG] cannot use dataframe as input for second level GLM #3871

Changes proposed in this pull request:

• add regression test
• implement fix suggested by gurraburra

[github-actions]

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[codecov]

Codecov Report

Merging #3879 (d41b0c2) into main (5734dba) will increase coverage by 0.00%.
Report is 5 commits behind head on main.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #3879   +/-   ##
=======================================
  Coverage   91.60%   91.61%           
=======================================
  Files         134      134           
  Lines       15713    15722    +9     
  Branches     3271     3273    +2     
=======================================
+ Hits        14394    14403    +9     
  Misses        770      770           
  Partials      549      549           

Flag	Coverage Δ
macos-latest_3.10	91.53% <100.00%> (+<0.01%)	⬆️
macos-latest_3.11	91.53% <100.00%> (+<0.01%)	⬆️
macos-latest_3.8	91.49% <100.00%> (+<0.01%)	⬆️
macos-latest_3.9	91.49% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.10	91.53% <100.00%> (?)
ubuntu-latest_3.11	91.53% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.8	91.49% <100.00%> (+<0.01%)	⬆️
ubuntu-latest_3.9	91.49% <100.00%> (+<0.01%)	⬆️
windows-latest_3.10	91.47% <100.00%> (+<0.01%)	⬆️
windows-latest_3.11	91.47% <100.00%> (+<0.01%)	⬆️
windows-latest_3.8	91.44% <100.00%> (?)
windows-latest_3.9	91.44% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
nilearn/glm/second_level/second_level.py	96.39% <100.00%> (-0.02%)	⬇️

... and 1 file with indirect coverage changes

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[bthirion]

LGTM, thx.