fix(module:dropdown): fix dropdown contextmenu

[vthinkxie]

close #3768

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?

[ ] 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
[ ] No

Other information

[codecov]

Codecov Report

Merging #3782 into master will decrease coverage by <.01%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           master    #3782      +/-   ##
==========================================
- Coverage   95.39%   95.39%   -0.01%     
==========================================
  Files         712      712              
  Lines       14543    14544       +1     
  Branches     1913     1914       +1     
==========================================
  Hits        13874    13874              
  Misses        243      243              
- Partials      426      427       +1

Impacted Files	Coverage Δ
components/dropdown/nz-context-menu.service.ts	95% <83.33%> (-1.62%)	⬇️

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