fix(Input): Display floating input label if value is not undefined

[cehsu]

This change ensures that floating labels appear for zero values.

Summary

Bug:

1. Blur out of an input with a floating label, and a value of 0 (can be tested via value={0}).
2. We expect the label to remain, but it doesn't:
   

Cause:

1. Zero values were not being accounted for in checking whether to display floating labels. Checking for zero values allows the label to be displayed:
   

[codecov]

Codecov Report

Merging #50 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master     #50   +/-   ##
======================================
  Coverage    68.5%   68.5%           
======================================
  Files          22      22           
  Lines         435     435           
  Branches       92      92           
======================================
  Hits          298     298           
  Misses        107     107           
  Partials       30      30

Impacted Files	Coverage Δ
src/Form/Label.js	33.33% <ø> (ø)	⬆️
src/Form/Input.js	46.55% <100%> (ø)	⬆️

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov]

Codecov Report

Merging #50 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master     #50   +/-   ##
======================================
  Coverage    68.5%   68.5%           
======================================
  Files          22      22           
  Lines         435     435           
  Branches       92      92           
======================================
  Hits          298     298           
  Misses        107     107           
  Partials       30      30

Impacted Files	Coverage Δ
src/Form/Label.js	33.33% <ø> (ø)	⬆️
src/Form/Input.js	46.55% <100%> (ø)	⬆️

---

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 a55b4a9...e46a982. Read the comment docs.