fix(module:pulltorefresh): fix when not use ngmodel worked error

[Guoyuanqiang]

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-mobile/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

[codecov]

Codecov Report

Merging #229 into master will decrease coverage by 0.09%.
The diff coverage is 37.5%.

@@            Coverage Diff            @@
##           master     #229     +/-   ##
=========================================
- Coverage   86.46%   86.36%   -0.1%     
=========================================
  Files         224      224             
  Lines        7298     7306      +8     
  Branches     1024     1032      +8     
=========================================
  Hits         6310     6310             
- Misses        588      590      +2     
- Partials      400      406      +6

Impacted Files	Coverage Δ
...nents/pull-to-refresh/pull-to-refresh.component.ts	73.33% <37.5%> (-3.74%)	⬇️

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