fix(module:select): option item not selected with falsy value

[diyews]

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?

nz-select selected falsy value (eg.0) doesn't have the selected class
https://stackblitz.com/edit/angular-8iudzt

Issue Number: N/A

What is the new behavior?

Selected falsy value should have the selected class

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #5264 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #5264      +/-   ##
==========================================
- Coverage   91.32%   91.31%   -0.02%     
==========================================
  Files         551      551              
  Lines       13908    13908              
  Branches     2099     2099              
==========================================
- Hits        12702    12700       -2     
- Misses        713      717       +4     
+ Partials      493      491       -2     

Impacted Files	Coverage Δ
components/select/option-item.component.ts	92.85% <100.00%> (ø)
components/table/src/table/tbody.component.ts	85.71% <0.00%> (-14.29%)	⬇️
components/table/src/table/tr-measure.component.ts	86.95% <0.00%> (-13.05%)	⬇️
components/time-picker/time-holder.ts	95.78% <0.00%> (+1.05%)	⬆️
components/core/util/text-measure.ts	91.30% <0.00%> (+1.73%)	⬆️

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[wenqi73]

@diyews Seems right to me, you should also add a unit test.

[vthinkxie]

lgtm

[wenqi73]

@diyews Thanks!

[diyews]

@wenqi73 I can provide an unit test commit if need.

[wenqi73]

@diyews That will be great!