fix(module:mention): fix cannot to switch trigger

[hsuanxyz]

close #3629

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
[ ] No

Other information

[netlify]

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Built with commit 4fbc147

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[codecov]

Codecov Report

Merging #3632 into master will decrease coverage by <.01%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master    #3632      +/-   ##
==========================================
- Coverage   95.46%   95.46%   -0.01%     
==========================================
  Files         685      687       +2     
  Lines       14080    14112      +32     
  Branches     1874     1879       +5     
==========================================
+ Hits        13442    13472      +30     
- Misses        226      227       +1     
- Partials      412      413       +1

Impacted Files	Coverage Δ
components/mention/nz-mention-trigger.ts	100% <100%> (ø)	⬆️
components/mention/public-api.ts	100% <100%> (ø)	⬆️
components/mention/nz-mention.component.ts	98.85% <91.66%> (-0.57%)	⬇️
components/mention/nz-mention.service.ts	92.3% <92.3%> (ø)
components/dropdown/nz-dropdown.directive.ts	88.95% <0%> (-2.24%)	⬇️
components/table/nz-table.component.ts	93.54% <0%> (-0.69%)	⬇️
components/grid/demo/responsive.ts	100% <0%> (ø)	⬆️
components/grid/demo/offset.ts	100% <0%> (ø)	⬆️
components/grid/demo/flex-order.ts	100% <0%> (ø)	⬆️
components/comment/demo/nested.ts	100% <0%> (ø)	⬆️
... and 16 more

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[vthinkxie]

LGTM