fix: (module: grid): fix return contentType error

[Guoyuanqiang]

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-mobile/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
[ ] No

Other information

[codecov]

Codecov Report

Merging #320 into master will not change coverage.
The diff coverage is 50%.

@@           Coverage Diff           @@
##           master     #320   +/-   ##
=======================================
  Coverage   89.63%   89.63%           
=======================================
  Files         229      229           
  Lines        6872     6872           
  Branches      561      561           
=======================================
  Hits         6160     6160           
  Misses        548      548           
  Partials      164      164

Impacted Files	Coverage Δ
components/grid/grid.component.ts	94.11% <50%> (ø)	⬆️

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