docs(module:tooltip etc): trigger could be null

[wzhudev]

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
[x] 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

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[codecov]

Codecov Report

Merging #4298 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4298   +/-   ##
=======================================
  Coverage   92.16%   92.16%           
=======================================
  Files         520      520           
  Lines       11016    11016           
  Branches     1997     1997           
=======================================
  Hits        10153    10153           
  Misses        431      431           
  Partials      432      432

Impacted Files	Coverage Δ
...mponents/tooltip/base/nz-tooltip-base.directive.ts	91.08% <ø> (ø)	⬆️

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[netlify]

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[vthinkxie]

LGTM