[WIP] Add NullOption, and use it for Filters

[jeffreythewang]

[WIP]

[codecov-io]

Codecov Report

Merging #8 into master will decrease coverage by <.01%.
The diff coverage is 70.83%.

@@            Coverage Diff             @@
##           master       #8      +/-   ##
==========================================
- Coverage   71.37%   71.37%   -0.01%     
==========================================
  Files         190      191       +1     
  Lines       14918    14938      +20     
  Branches     1102     1105       +3     
==========================================
+ Hits        10648    10662      +14     
- Misses       4267     4273       +6     
  Partials        3        3

Impacted Files	Coverage Δ
...ipts/explore/components/controls/SelectControl.jsx	84.74% <100%> (ø)	⬆️
...erset/assets/javascripts/components/NullOption.jsx	100% <100%> (ø)
.../assets/javascripts/components/AlteredSliceTag.jsx	100% <100%> (ø)	⬆️
...ipts/explore/components/controls/FilterControl.jsx	97.29% <100%> (ø)	⬆️
...uperset/assets/javascripts/explore/exploreUtils.js	76.82% <16.66%> (-3.44%)	⬇️
...javascripts/explore/components/controls/Filter.jsx	77.94% <60%> (-1.43%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ec06967...18eb303. Read the comment docs.