fix: fix TypeError on range slider when min equals max

[tlambert03]

fixes #307

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.40%. Comparing base (b11b511) to head (301d233).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #310      +/-   ##
==========================================
+ Coverage   86.38%   86.40%   +0.02%     
==========================================
  Files          49       49              
  Lines        3862     3862              
==========================================
+ Hits         3336     3337       +1     
+ Misses        526      525       -1     

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[Czaki]

looks ok

[tlambert03]

Thanks for quick review @Czaki!