[4.1] Fix latest rubocop offenses

[tvdeyen]

Same as #5653 but for 4.1 branch

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (9394f9f) 88.74% compared to head (042193c) 88.74%.

Files	Patch %	Lines
core/app/models/spree/promotion_handler/coupon.rb	0.00%	1 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##             v4.1    #5654   +/-   ##
=======================================
  Coverage   88.74%   88.74%           
=======================================
  Files         563      563           
  Lines       13883    13883           
=======================================
  Hits        12320    12320           
  Misses       1563     1563           

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