fix(module: ActionSheet): fix when no title or message would not show blank on header

[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?

[ ] 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
[x] 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 #192 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #192      +/-   ##
==========================================
- Coverage   86.24%   86.22%   -0.03%     
==========================================
  Files         222      222              
  Lines        7156     7158       +2     
  Branches     1002     1003       +1     
==========================================
  Hits         6172     6172              
- Misses        587      588       +1     
- Partials      397      398       +1

Impacted Files	Coverage Δ
components/action-sheet/action-sheet.component.ts	79.16% <100%> (-7.2%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 23f36c1...5752715. Read the comment docs.

[Guoyuanqiang]

close #172