[Fixes #5049] Thumbnails are generated with a point style for line/polygon layers

[afabiani]

• Fixes Thumbnails are generated with a point style for line/polygon layers #5049 : Thumbnails are generated with a point style for line/polygon layers

Checklist

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[codecov]

Codecov Report

Merging #5116 into master will increase coverage by 0.09%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master    #5116      +/-   ##
==========================================
+ Coverage    53.6%   53.69%   +0.09%     
==========================================
  Files         185      185              
  Lines       20791    20800       +9     
  Branches     3301     3306       +5     
==========================================
+ Hits        11144    11169      +25     
+ Misses       8637     8622      -15     
+ Partials     1010     1009       -1

[capooti]

Thanks @afabiani for adding the tests! great