fix(datepicker): allow empty string as placeholder

[arthurdenner]

What kind of change does this PR introduce?
Bug fix. Closes #260.

What is the current behavior?
placeholder prop is overridden with it's an empty string or null.

What is the new behavior?
If the placeholder prop is different from undefined, it's applied. Otherwise, we use the format prop as placeholder.

Checklist:

• Documentation
• Tests
• Ready to be merged

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[codecov]

Codecov Report

Merging #261 into develop will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #261   +/-   ##
========================================
  Coverage    87.25%   87.25%           
========================================
  Files           14       14           
  Lines          463      463           
  Branches        92       92           
========================================
  Hits           404      404           
  Misses          56       56           
  Partials         3        3           

Impacted Files	Coverage Δ
src/index.tsx	90.52% <100.00%> (ø)

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[arthurdenner]

🎉 This PR is included in version 2.5.3 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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