fix(module:mention): not emit (nzOnSearchChange) when value is empty

[hsuanxyz]

close #5722

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?

Issue Number: #5722

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[codecov]

Codecov Report

Merging #5729 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5729   +/-   ##
=======================================
  Coverage   90.06%   90.07%           
=======================================
  Files         455      455           
  Lines       13827    13827           
  Branches     2183     2183           
=======================================
+ Hits        12453    12454    +1     
  Misses        847      847           
+ Partials      527      526    -1     

Impacted Files	Coverage Δ
components/mention/mention.component.ts	98.79% <100.00%> (ø)
components/time-picker/time-holder.ts	95.78% <0.00%> (+1.05%)	⬆️

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[github-actions]

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