fix(module:modal): nzModalFooter not work when the modal open on init

[hsuanxyz]

close #4948

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: #4948

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[codecov]

Codecov Report

Merging #4954 into master will increase coverage by 0.03%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #4954      +/-   ##
==========================================
+ Coverage   91.55%   91.59%   +0.03%     
==========================================
  Files         535      535              
  Lines       13688    13687       -1     
  Branches     2043     2044       +1     
==========================================
+ Hits        12532    12536       +4     
+ Misses        693      687       -6     
- Partials      463      464       +1     

Impacted Files	Coverage Δ
components/modal/modal.component.ts	96.84% <71.42%> (+2.05%)	⬆️
components/core/util/text-measure.ts	89.56% <0.00%> (-1.74%)	⬇️
components/table/src/table/tr-measure.component.ts	100.00% <0.00%> (+13.04%)	⬆️
components/table/src/table/tbody.component.ts	100.00% <0.00%> (+14.28%)	⬆️

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