perf(module:checkbox): use css empty selector instead of observeContent

[vthinkxie]

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

[zorro-bot]

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[codecov]

Codecov Report

Merging #4761 into master will decrease coverage by 0.04%.
The diff coverage is 90.47%.

@@            Coverage Diff             @@
##           master    #4761      +/-   ##
==========================================
- Coverage   93.26%   93.21%   -0.05%     
==========================================
  Files         506      506              
  Lines       12776    12757      -19     
  Branches     2015     1964      -51     
==========================================
- Hits        11915    11891      -24     
- Misses        451      453       +2     
- Partials      410      413       +3

Impacted Files	Coverage Δ
components/checkbox/checkbox.module.ts	100% <ø> (ø)
components/checkbox/checkbox-wrapper.component.ts	100% <100%> (ø)
components/checkbox/checkbox-group.component.ts	100% <100%> (ø)
components/checkbox/checkbox.component.ts	96.29% <80%> (ø)
components/core/util/check.ts	79.06% <0%> (-4.66%)	⬇️
components/time-picker/time-holder.ts	97.31% <0%> (-0.68%)	⬇️

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