Define padded mixing arr if skip_vols are 0

[tsalo]

Closes none. This fixes a bug where ica_aroma_confound_extraction will fail if skip_vols is 0.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 30.85%. Comparing base (99bf9ca) to head (eed7389).
Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
src/fmripost_aroma/interfaces/confounds.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #69      +/-   ##
==========================================
- Coverage   30.87%   30.85%   -0.02%     
==========================================
  Files          26       26              
  Lines        2329     2330       +1     
  Branches      365      365              
==========================================
  Hits          719      719              
- Misses       1589     1590       +1     
  Partials       21       21              

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