fix(module:tooltip): keep tooltip visibility when trigger is null

[stygian-desolator]

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

close #6483
close #6402

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[zorro-bot]

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[codecov]

Codecov Report

Merging #6489 (19275ac) into master (0e326e7) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #6489   +/-   ##
=======================================
  Coverage   89.83%   89.83%           
=======================================
  Files         478      478           
  Lines       15314    15314           
  Branches     2332     2332           
=======================================
  Hits        13758    13758           
+ Misses        939      938    -1     
- Partials      617      618    +1     

Impacted Files	Coverage Δ
components/tooltip/base.ts	91.98% <100.00%> (+0.94%)	⬆️
components/core/util/text-measure.ts	89.56% <0.00%> (-1.74%)	⬇️

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[wzhudev]

LGTM. Thanks!

[Aizaz-Ulhaq]

The issue is still there. It's not solved (as of documentation page). please look at the attached .gif from the documentation page. Clicking anywhere on the page or on the scroller still hides the tooltip.