Added filtering undefined values in array of SelectField (fixes #691).

[kestarumper]

This PR introduces filtering undefined values inside SelectField to fix empty boxes.

[codecov]

Codecov Report

Merging #695 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #695   +/-   ##
=======================================
  Coverage   98.34%   98.34%           
=======================================
  Files         163      163           
  Lines        1810     1812    +2     
  Branches      562      563    +1     
=======================================
+ Hits         1780     1782    +2     
  Misses         10       10           
  Partials       20       20           

Impacted Files	Coverage Δ
packages/uniforms-antd/src/SelectField.tsx	100.00% <100.00%> (ø)

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