[BUG] fix plot_crossplot_std for predict_var return

[fkiraly]

This PR fixes an unreported bug where plot_crossplot_std would crash for for a predict_var return (as opposed to a full distributional prediction).

This is fixed, and a test is added.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.03% 🎉

Comparison is base (54a73a1) 71.35% compared to head (3acc465) 71.39%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #71      +/-   ##
==========================================
+ Coverage   71.35%   71.39%   +0.03%     
==========================================
  Files          92       92              
  Lines        4940     4943       +3     
  Branches      897      897              
==========================================
+ Hits         3525     3529       +4     
  Misses       1179     1179              
+ Partials      236      235       -1     

Files Changed	Coverage Δ
skpro/utils/plotting.py	94.73% <100.00%> (+1.87%)	⬆️
skpro/utils/tests/test_plots.py	100.00% <100.00%> (ø)

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