fix(module:cascader): add nzClear functionality to cascader

[chriso86]

Add the missing nzClear emit to the clearSelection method on the cascader component.
Extend the cascader e2e tests to ensure that the event emitter works as expected.

fix #6751

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/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?

The nzClear event emitter would not emit when the cascader value was cleared as the implementation was missing.

Issue Number: #6751

What is the new behavior?

When the clearSelection method is executed on the cascader component, the nzClear event emitter will now emit to inform any cascader component consumers.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Used .spyOn and .toHaveBeenCalled to test the event emitter in the e2e spec for the cascader file.
Tests are passing.

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #6761 (c080f1a) into master (4f915b8) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #6761      +/-   ##
==========================================
+ Coverage   89.92%   89.95%   +0.03%     
==========================================
  Files         480      480              
  Lines       15487    15487              
  Branches     2516     2516              
==========================================
+ Hits        13926    13931       +5     
+ Misses        940      936       -4     
+ Partials      621      620       -1     

Impacted Files	Coverage Δ
components/cascader/cascader.service.ts	97.56% <ø> (-0.02%)	⬇️
components/cascader/cascader.component.ts	94.67% <100.00%> (+0.01%)	⬆️
components/tabs/tab-nav-bar.component.ts	83.74% <0.00%> (+0.70%)	⬆️
components/table/src/table-data.service.ts	85.24% <0.00%> (+4.91%)	⬆️

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[chriso86]

I should have force pushed the last branch but I deleted the remote branch. 😞 So I had to recreate the PR. Sorry about that.

[wzhudev]

I should have force pushed the last branch but I deleted the remote branch. 😞 So I had to recreate the PR. Sorry about that.

Not a problem. Thanks for your contribution~!

[wzhudev]

LGTM

[chriso86]

You're welcome!