fix(module: date-picker): fix NAN bug when fast scroll

[BronzeCui]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd-mobile/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Application (the showcase website) / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

fix #579

[codecov]

Codecov Report

Merging #589 into master will decrease coverage by 0.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #589      +/-   ##
==========================================
- Coverage   88.39%   88.38%   -0.02%     
==========================================
  Files         270      270              
  Lines        7599     7600       +1     
  Branches     1083     1084       +1     
==========================================
  Hits         6717     6717              
- Misses        435      436       +1     
  Partials      447      447

Impacted Files	Coverage Δ
components/date-picker/date-picker.component.ts	73.7% <0%> (-0.15%)	⬇️

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[DeMeng33]

thanks