Fix optional annotation affecting later widgets

[tlambert03]

fixes #261.
was using a mutable default in the function signature

[codecov]

Codecov Report

Merging #262 (dc5c46d) into master (fd99155) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #262      +/-   ##
==========================================
+ Coverage   90.68%   90.70%   +0.01%     
==========================================
  Files          28       28              
  Lines        3103     3108       +5     
==========================================
+ Hits         2814     2819       +5     
  Misses        289      289              

Impacted Files	Coverage Δ
magicgui/type_map.py	91.32% <100.00%> (+0.15%)	⬆️
magicgui/widgets/_bases/categorical_widget.py	85.29% <100.00%> (+0.21%)	⬆️
magicgui/widgets/_bases/create_widget.py	97.22% <100.00%> (+0.07%)	⬆️

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