fix(module:tooltip): empty judgement

[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?

[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

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[codecov]

Codecov Report

Merging #3993 into master will decrease coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #3993     +/-   ##
=========================================
- Coverage   93.25%   93.15%   -0.1%     
=========================================
  Files         500      500             
  Lines       10615    10614      -1     
  Branches     1924     1924             
=========================================
- Hits         9899     9888     -11     
- Misses        301      310      +9     
- Partials      415      416      +1

Impacted Files	Coverage Δ
...s/tooltip/base/nz-tooltip-base-legacy.component.ts	96.15% <ø> (-0.15%)	⬇️
...mponents/tooltip/base/nz-tooltip-base.component.ts	94.44% <100%> (+2.77%)	⬆️
components/popover/nz-popover.component.ts	100% <100%> (ø)	⬆️
components/tabs/nz-tabs-ink-bar.directive.ts	60% <0%> (-30%)	⬇️
components/rate/nz-rate.component.ts	93.1% <0%> (-4.6%)	⬇️
components/time-picker/time-holder.ts	96.49% <0%> (-0.88%)	⬇️

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