fix: Serial UpdateStrategy not working without roles

[free6om]

fixed #6785

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 65.54%. Comparing base (654f3bb) to head (4ba8693).

Files	Patch %	Lines
pkg/controller/rsm/update_plan.go	33.33%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           release-0.8    #7079      +/-   ##
===============================================
+ Coverage        65.49%   65.54%   +0.04%     
===============================================
  Files              306      306              
  Lines            38066    38069       +3     
===============================================
+ Hits             24932    24952      +20     
+ Misses           10986    10971      -15     
+ Partials          2148     2146       -2     

Flag	Coverage Δ
unittests	65.54% <33.33%> (+0.04%)	⬆️

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