fix(module:mention): page not loading entirely

[ParsaArvanehPA]

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?

Currently when you navigate to mention component, you'll notice that mention page does not load at all. This behaviour is caused by a logical error in form.ts.
At line number 54, this.mentionChild is equal to undefined. I just added an optional chain so if it is undefined it won't throw an error.

Issue Number: N/A

What is the new behavior?

Mention page can now be loaded and viewed, and form.ts has the same functionality as intended (checked with version 15 of ng-zorro).

Does this PR introduce a breaking change?

[ ] Yes
[✔] No

Other information

Image of mention component not working:

[zorro-bot]

This preview will be available after the AzureCI is passed.

[codecov]

Codecov Report

Merging #8146 (63414af) into master (50f6700) will decrease coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 63414af differs from pull request most recent head 16bc62c. Consider uploading reports for the commit 16bc62c to get more accurate results

@@            Coverage Diff             @@
##           master    #8146      +/-   ##
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- Coverage   91.69%   91.69%   -0.01%     
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  Files         517      517              
  Lines       17795    17795              
  Branches     2724     2724              
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- Hits        16318    16317       -1     
- Misses       1178     1179       +1     
  Partials      299      299              

see 1 file with indirect coverage changes