fix(module:grid): nzGutter support string type

[cipchk]

The following code style is better in strictTemplates mode:

<nz-row [nzGutter]="16">

better:

<nz-row nzGutter="16">

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: N/A

What is the new behavior?

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 #6450 (cdc8c6f) into master (2200adc) will decrease coverage by 0.04%.
The diff coverage is 91.30%.

@@            Coverage Diff             @@
##           master    #6450      +/-   ##
==========================================
- Coverage   89.86%   89.81%   -0.05%     
==========================================
  Files         478      478              
  Lines       15277    15314      +37     
  Branches     2326     2475     +149     
==========================================
+ Hits        13729    13755      +26     
- Misses        930      941      +11     
  Partials      618      618              

Impacted Files	Coverage Δ
components/core/config/config.ts	100.00% <ø> (ø)
components/message/message.component.ts	100.00% <ø> (ø)
components/notification/notification.component.ts	100.00% <ø> (ø)
components/table/src/table/thead.component.ts	91.22% <ø> (ø)
...ponents/time-picker/time-picker-panel.component.ts	88.70% <ø> (ø)
components/transfer/transfer.component.ts	91.73% <ø> (ø)
components/table/src/addon/filter.component.ts	60.31% <50.00%> (+1.85%)	⬆️
components/message/base.ts	87.39% <72.72%> (-1.60%)	⬇️
...mponents/date-picker/date-range-popup.component.ts	91.39% <95.23%> (+0.54%)	⬆️
components/date-picker/date-picker.component.ts	93.42% <100.00%> (+0.04%)	⬆️
... and 17 more

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