fix(module:form): optimize code to increase robustness

[danranVm]

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
[x] 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]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #5550 into master will decrease coverage by 0.02%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master    #5550      +/-   ##
==========================================
- Coverage   91.22%   91.19%   -0.03%     
==========================================
  Files         454      454              
  Lines       13953    13954       +1     
  Branches     2219     2219              
==========================================
- Hits        12728    12726       -2     
- Misses        719      720       +1     
- Partials      506      508       +2     

Impacted Files	Coverage Δ
components/form/form-control.component.ts	94.26% <0.00%> (-0.82%)	⬇️
components/form/form.directive.ts	100.00% <100.00%> (ø)
components/core/util/text-measure.ts	89.56% <0.00%> (-1.74%)	⬇️

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[vthinkxie]

lgtm