fix: Listify sessions when generating reports

[effigies]

Closes #3407.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 48.21%. Comparing base (0218f4f) to head (f1a67c0).
Report is 4 commits behind head on maint/24.1.x.

Files with missing lines	Patch %	Lines
fmriprep/reports/core.py	0.00%	1 Missing and 1 partial ⚠️

❗ There is a different number of reports uploaded between BASE (0218f4f) and HEAD (f1a67c0). Click for more details.

HEAD has 4 uploads less than BASE

Flag	BASE (0218f4f)	HEAD (f1a67c0)
	7	3

Additional details and impacted files

@@                Coverage Diff                @@
##           maint/24.1.x    #3408       +/-   ##
=================================================
- Coverage         71.78%   48.21%   -23.57%     
=================================================
  Files                57       57               
  Lines              4256     4258        +2     
  Branches            458      459        +1     
=================================================
- Hits               3055     2053     -1002     
- Misses             1086     2135     +1049     
+ Partials            115       70       -45     

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