Handle empty ignore settings

[janbrummer]

Fixes: #299

[codecov]

Codecov Report

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

Project coverage is 69.13%. Comparing base (57abe9b) to head (ff720e5).

Files	Patch %	Lines
src/backend/px-manager.c	0.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #300   +/-   ##
=======================================
  Coverage   69.13%   69.13%           
=======================================
  Files          18       18           
  Lines         878      878           
  Branches      258      258           
=======================================
  Hits          607      607           
  Misses        164      164           
  Partials      107      107           

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