fix(module:select): fix select scroll bottom not emit with search

[vthinkxie]

close #3777

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?

[ ] 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
[ ] No

Other information

[codecov]

Codecov Report

Merging #4272 into master will decrease coverage by 0.03%.
The diff coverage is 40%.

@@            Coverage Diff             @@
##           master    #4272      +/-   ##
==========================================
- Coverage   92.21%   92.18%   -0.04%     
==========================================
  Files         520      520              
  Lines       10982    10986       +4     
  Branches     1990     1990              
==========================================
  Hits        10127    10127              
- Misses        427      430       +3     
- Partials      428      429       +1

Impacted Files	Coverage Δ
components/select/nz-option-container.component.ts	88.88% <40%> (-7.99%)	⬇️
components/dropdown/nz-dropdown.component.ts	93.75% <0%> (-2.09%)	⬇️

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