BF: Have getPaths return None if filePath argument is not a string

[tpronk]

This bug-fix tackles an issue that was reported on the forum: https://discourse.psychopy.org/t/unhandled-error-no-new-js-file-being-created/21882/7?u=thomas_pronk

[codecov]

Codecov Report

Merging #3719 (a2cfe68) into release (336cdd7) will decrease coverage by 0.00%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           release    #3719      +/-   ##
===========================================
- Coverage    43.24%   43.24%   -0.01%     
===========================================
  Files          267      267              
  Lines        52798    52804       +6     
  Branches      9108     9110       +2     
===========================================
+ Hits         22831    22833       +2     
- Misses       27647    27650       +3     
- Partials      2320     2321       +1     

Impacted Files	Coverage Δ
psychopy/experiment/_experiment.py	52.03% <0.00%> (-0.19%)	⬇️
psychopy/app/connections/updates.py	13.39% <0.00%> (-0.07%)	⬇️
psychopy/data/trial.py	68.70% <0.00%> (ø)
psychopy/experiment/params.py	82.23% <0.00%> (+0.23%)	⬆️

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[coveralls]

Coverage increased (+0.0001%) to 47.086% when pulling a2cfe68 on tpronk:release into 5d7df1b on psychopy:release.