Fix empty dict as default arg

[eudoxos]

See e.g. https://towardsdatascience.com/python-pitfall-mutable-default-arguments-9385e8265422

(It happened to me that an object was stubbornly getting color set in previous runs.)

[codecov]

Codecov Report

Merging #322 (14146f8) into master (0d091ba) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #322   +/-   ##
=======================================
  Coverage   88.59%   88.59%           
=======================================
  Files          19       19           
  Lines        1490     1491    +1     
  Branches      201      201           
=======================================
+ Hits         1320     1321    +1     
  Misses        137      137           
  Partials       33       33           

Impacted Files	Coverage Δ
cq_editor/widgets/object_tree.py	93.92% <100.00%> (+0.02%)	⬆️

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[adam-urbanczyk]

Thanks, how did you manage to modify the dict? Or did it happen in the CQ-editor code?

[eudoxos]

Thanks, how did you manage to modify the dict? \

I can't remember anymore; I was trying different ways to make show_object show the object with label (and persistent properties) in the CQ editor; and then it happened that re-running the script with options removed was still setting the color.