fix(module:dropdown): fix dropdown style conflict with material style

[vthinkxie]

close #3241

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

[netlify]

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Built with commit 7f86f28

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[codecov]

Codecov Report

Merging #3290 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3290   +/-   ##
=======================================
  Coverage   97.38%   97.38%           
=======================================
  Files         559      559           
  Lines       11650    11650           
  Branches      837      837           
=======================================
  Hits        11345    11345           
  Misses        194      194           
  Partials      111      111

Impacted Files	Coverage Δ
components/dropdown/nz-dropdown.service.ts	100% <ø> (ø)	⬆️

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