fix: fix 'null' is not assignable to type 'NzFormControlStatusType'

[cipchk]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ng-alain/delon/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
[x] No

Other information

[ng-alain-bot]

Preview is ready!

[codecov]

Codecov Report

Merging #1502 (d3ea62f) into master (5638156) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1502   +/-   ##
=======================================
  Coverage   95.37%   95.37%           
=======================================
  Files         270      270           
  Lines        8436     8436           
  Branches     1575     1575           
=======================================
  Hits         8046     8046           
  Misses        300      300           
  Partials       90       90           

Impacted Files	Coverage Δ
packages/form/src/model/form.property.ts	97.50% <100.00%> (ø)
packages/form/src/sf.component.ts	95.89% <100.00%> (ø)

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