PATH: do not accept empty strings, fixes #4221

[ThomasWaldmann]

fwd port of #8005

[codecov-commenter]

Codecov Report

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

Comparison is base (6a511fd) 83.65% compared to head (73284db) 83.66%.
Report is 4 commits behind head on master.

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           @@
##           master    #8017   +/-   ##
=======================================
  Coverage   83.65%   83.66%           
=======================================
  Files          66       66           
  Lines       11860    11866    +6     
  Branches     2149     2150    +1     
=======================================
+ Hits         9922     9928    +6     
  Misses       1363     1363           
  Partials      575      575           

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