fix(module:date-picker): fix datePicker can't clear

[luckyship]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x ] The commit message follows our guidelines: https://github.com/NG-ZORRO/ng-zorro-antd/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: 7534

What is the new behavior?

it can clear range-picker when it has default value

Does this PR introduce a breaking change?

[ ] Yes
[x ] No

Other information

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #7671 (7c4bb28) into master (16e61be) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #7671   +/-   ##
=======================================
  Coverage   92.03%   92.04%           
=======================================
  Files         502      502           
  Lines       16751    16752    +1     
  Branches     2634     2634           
=======================================
+ Hits        15417    15419    +2     
  Misses       1064     1064           
+ Partials      270      269    -1     

Impacted Files	Coverage Δ
components/date-picker/date-picker.component.ts	93.71% <100.00%> (+0.01%)	⬆️
components/core/util/text-measure.ts	92.10% <0.00%> (-0.88%)	⬇️
components/tabs/tab-nav-bar.component.ts	88.65% <0.00%> (+0.70%)	⬆️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

[wenqi73]

@luckyship Could you add a unit test for this?

[luckyship]

@luckyship Could you add a unit test for this?

sorry, let me add new unit test for this

[luckyship]

@wenqi73 i have added new unit test, please check again, thanks

[wenqi73]

@luckyship Thanks!