fix(module:date-picker): fix select error when ngmodel set to null

[nuonuoge]

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

[codecov]

Codecov Report

Merging #310 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #310      +/-   ##
==========================================
+ Coverage   89.64%   89.65%   +0.01%     
==========================================
  Files         229      229              
  Lines        6864     6864              
  Branches      561      560       -1     
==========================================
+ Hits         6153     6154       +1     
+ Misses        547      546       -1     
  Partials      164      164

Impacted Files	Coverage Δ
components/date-picker/date-picker.component.ts	79.62% <100%> (+0.23%)	⬆️

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