fix(module: input-item): fix input value can't be 0 or ''

[nuonuoge]

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 #185 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #185      +/-   ##
==========================================
+ Coverage   86.25%   86.33%   +0.07%     
==========================================
  Files         222      222              
  Lines        7161     7162       +1     
  Branches     1003     1003              
==========================================
+ Hits         6177     6183       +6     
+ Misses        586      583       -3     
+ Partials      398      396       -2

Impacted Files	Coverage Δ
components/input-item/input-item.component.ts	95.18% <100%> (+2.71%)	⬆️

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