Only render the thumbnails if there are canvas groupings in the first place

[cbeer]

This incidentally fixes a problem with the first item in the canvas navigation, which seems like it might start to render before we have dimension information and we use the default width instead of the canvas width:

Before:

After/correct:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.51%. Comparing base (644b804) to head (816eef8).
Report is 4 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4063   +/-   ##
=======================================
  Coverage   94.51%   94.51%           
=======================================
  Files         313      313           
  Lines       14752    14753    +1     
  Branches     2486     2487    +1     
=======================================
+ Hits        13943    13944    +1     
  Misses        804      804           
  Partials        5        5           

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