fix(datepicker): make it valid in case of empty input

[Tibing]

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• I read and followed the CONTRIBUTING.md guide.
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Short description of what this resolves:

Closes #1182

[codecov]

Codecov Report

Merging #1247 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1247      +/-   ##
==========================================
+ Coverage    79.3%   79.47%   +0.16%     
==========================================
  Files         232      232              
  Lines        7017     7019       +2     
  Branches      586      587       +1     
==========================================
+ Hits         5565     5578      +13     
+ Misses       1289     1272      -17     
- Partials      163      169       +6

Impacted Files	Coverage Δ
...heme/components/datepicker/datepicker.directive.ts	70.49% <100%> (+8.82%)	⬆️
...heme/components/datepicker/datepicker.component.ts	81.54% <0%> (+0.59%)	⬆️

[nnixaa]

@Tibing can we add a simple test?