Update timepicker.reducer.ts

[chonggit]

fix if state.value == null _newState.value will set to '1970-01-01 08:00:00'

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• read and followed the CONTRIBUTING.md guide.
• built and tested the changes locally.
• added/updated tests.
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[codecov]

Codecov Report

Merging #3207 into development will decrease coverage by 0.01%.
The diff coverage is 50%.

@@               Coverage Diff               @@
##           development    #3207      +/-   ##
===============================================
- Coverage         65.2%   65.19%   -0.02%     
===============================================
  Files              209      209              
  Lines             5653     5654       +1     
  Branches           996      997       +1     
===============================================
  Hits              3686     3686              
  Misses            1704     1704              
- Partials           263      264       +1

Impacted Files	Coverage Δ
src/timepicker/reducer/timepicker.reducer.ts	89.18% <50%> (-2.48%)	⬇️

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[YevheniiaMazur]

Tested, looks good