Fix subject collection (again)

[tsalo]

Stems from #77. The fix in #77 doesn't work when config.execution.bids_filters is None.

[codecov]

Codecov Report

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

Project coverage is 30.59%. Comparing base (062a726) to head (b80a99e).
Report is 12 commits behind head on main.

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

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #78   +/-   ##
=======================================
  Coverage   30.59%   30.59%           
=======================================
  Files          26       26           
  Lines        2363     2363           
  Branches      374      374           
=======================================
  Hits          723      723           
  Misses       1617     1617           
  Partials       23       23           

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