Fix result cleanup when retention period configured on job group level is zero

[Martchus]

See https://progress.opensuse.org/issues/91157

[codecov]

Codecov Report

Merging #3845 (45af066) into master (8b231d7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3845   +/-   ##
=======================================
  Coverage   96.75%   96.76%           
=======================================
  Files         368      368           
  Lines       32683    32693   +10     
=======================================
+ Hits        31624    31634   +10     
  Misses       1059     1059           

Impacted Files	Coverage Δ
t/37-limit_assets.t	100.00% <ø> (ø)
lib/OpenQA/Schema/Result/JobGroups.pm	99.40% <100.00%> (+0.01%)	⬆️
t/36-job_group_settings.t	100.00% <100.00%> (ø)

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