Fix check for Optional Type

[tlambert03]

fixes #269

[codecov]

Codecov Report

Merging #270 (1711a8e) into master (481f665) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #270   +/-   ##
=======================================
  Coverage   90.28%   90.28%           
=======================================
  Files          28       28           
  Lines        3182     3182           
=======================================
  Hits         2873     2873           
  Misses        309      309           

Impacted Files	Coverage Δ
magicgui/type_map.py	91.32% <100.00%> (ø)

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