fix(module:tabs): fix dropdown style

[hsuanxyz]

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?

Issue Number: N/A

What is the new behavior?

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 #5659 into master will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5659      +/-   ##
==========================================
+ Coverage   90.06%   90.07%   +0.01%     
==========================================
  Files         455      455              
  Lines       13798    13798              
  Branches     2183     2183              
==========================================
+ Hits        12427    12429       +2     
+ Misses        846      844       -2     
  Partials      525      525              

Impacted Files	Coverage Δ
components/tabs/tab-nav-operation.component.ts	62.06% <ø> (ø)
components/core/util/text-measure.ts	89.56% <0.00%> (-1.74%)	⬇️
components/tabs/tab-nav-bar.component.ts	82.33% <0.00%> (+1.41%)	⬆️

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