fix(module: modal): fix no actions would work 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
[x] No

Other information

[codecov]

Codecov Report

Merging #222 into master will increase coverage by 0.05%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master     #222      +/-   ##
==========================================
+ Coverage   86.45%   86.51%   +0.05%     
==========================================
  Files         224      224              
  Lines        7308     7303       -5     
  Branches     1029     1023       -6     
==========================================
  Hits         6318     6318              
  Misses        587      587              
+ Partials      403      398       -5

Impacted Files	Coverage Δ
components/modal/modal.service.ts	69.14% <50%> (+3.49%)	⬆️

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