fix(module:code-editor): only emit update if value changed

[Airblader]

Before notifying Angular (through the ControlValueAccessor mechanism) of a change to
the value we make sure that the value has actually changed. This is necessary to
avoid feedback loops as writeValue (model -> view) would otherwise cause this to be
fired (view -> model) which can cause infinite loop issues on the usage side or
broken validation detection.

fixes #5869

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) (Note: there currently are no tests for code-editor)
• Docs have been added / updated (for bug fixes / features) (Note: not required)

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: #5869

Does this PR introduce a breaking change?

[ ] Yes
[X] No

[codecov]

Codecov Report

Merging #5933 into master will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #5933      +/-   ##
==========================================
- Coverage   90.02%   90.02%   -0.01%     
==========================================
  Files         463      463              
  Lines       13947    13949       +2     
  Branches     2202     2203       +1     
==========================================
+ Hits        12556    12557       +1     
- Misses        856      857       +1     
  Partials      535      535              

Impacted Files	Coverage Δ
components/code-editor/code-editor.component.ts	13.72% <50.00%> (+0.72%)	⬆️

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[wzhudev]

LGTM

[github-actions]

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