PATH: do not accept empty strings, fixes #4221

[ThomasWaldmann]

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[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (60dcd53) 82.33% compared to head (8ad7a9a) 82.33%.
Report is 5 commits behind head on 1.4-maint.

Files	Patch %	Lines
src/borg/helpers/parseformat.py	50.00%	1 Missing and 1 partial ⚠️

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Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint    #8005      +/-   ##
=============================================
- Coverage      82.33%   82.33%   -0.01%     
=============================================
  Files             38       38              
  Lines          10781    10785       +4     
  Branches        2063     2064       +1     
=============================================
+ Hits            8877     8880       +3     
- Misses          1352     1353       +1     
  Partials         552      552              

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