Fix(NumberPicker): set null to reset value

[bindoon]

support null to reset NumberPicker

[codecov]

Codecov Report

Merging #782 into master will increase coverage by 6.59%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #782      +/-   ##
==========================================
+ Coverage   84.72%   91.31%   +6.59%     
==========================================
  Files         244      244              
  Lines       11245    14013    +2768     
  Branches     2806     4399    +1593     
==========================================
+ Hits         9527    12796    +3269     
- Misses        913     1198     +285     
+ Partials      805       19     -786

Impacted Files	Coverage Δ
src/number-picker/index.jsx	96.72% <100%> (+8.25%)	⬆️
src/menu/view/radio-item.jsx	66.66% <0%> (-33.34%)	⬇️
src/menu/view/checkbox-item.jsx	75% <0%> (-25%)	⬇️
src/util/dom.js	87.87% <0%> (-12.13%)	⬇️
src/tab/tabs/utils.js	86.95% <0%> (-6.38%)	⬇️
src/table/fixed/body.jsx	66.66% <0%> (-6.07%)	⬇️
src/table/base/resize.jsx	90% <0%> (-5.24%)	⬇️
src/calendar/head/card-header.jsx	80.64% <0%> (-2.69%)	⬇️
src/slider/slick/mixins/trackHelper.js	86.36% <0%> (-2.68%)	⬇️
src/upload/runtime/html5-uploader.jsx	88.23% <0%> (-2.68%)	⬇️
... and 207 more

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