Return empty array when Array knob is empty

[gpittarelli]

Issue: #1404

What I did

Made the array knob return an empty array instead of [""] when the input textbox is empty. Also added a test case.

How to test

Run the new test case.

Is this testable with jest or storyshots? jest

Does this need a new example in the kitchen sink apps? nah

Does this need an update to the documentation? probably not...

If your answer is yes to any of these, please make sure to include it in your PR.

[codecov]

Codecov Report

Merging #1811 into release/3.3 will increase coverage by 0.03%.
The diff coverage is 100%.

@@               Coverage Diff               @@
##           release/3.3    #1811      +/-   ##
===============================================
+ Coverage         23.3%   23.34%   +0.03%     
===============================================
  Files              277      277              
  Lines             6038     6041       +3     
  Branches           698      698              
===============================================
+ Hits              1407     1410       +3     
- Misses            4132     4140       +8     
+ Partials           499      491       -8

Impacted Files	Coverage Δ
addons/knobs/src/components/types/Array.js	84.61% <100%> (+4.61%)	⬆️
addons/info/src/components/types/OneOf.js	14.28% <0%> (ø)	⬆️
lib/ui/src/modules/ui/configs/init_panels.js	25% <0%> (ø)	⬆️
lib/ui/src/modules/ui/containers/addon_panel.js	23.52% <0%> (ø)	⬆️
...ents/stories_panel/stories_tree/tree_decorators.js	34.37% <0%> (ø)	⬆️
lib/components/src/table/cell.js	0% <0%> (ø)	⬆️
app/react/src/client/preview/element_check.js	41.17% <0%> (ø)	⬆️
addons/storyshots/src/require_context.js	45.58% <0%> (ø)	⬆️
lib/ui/src/modules/ui/components/layout/usplit.js	38.7% <0%> (ø)	⬆️
addons/info/src/components/types/PrettyPropType.js	10.2% <0%> (ø)	⬆️
... and 21 more

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[Hypnosphi]

Definitely an improvement, thanks!