fix(module:dropdown): fix dropdown ul list-style

[vthinkxie]

close #3268

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?

[ ] 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

[netlify]

Deploy preview for ng-zorro-master ready!

Built with commit f42f13a

https://deploy-preview-3284--ng-zorro-master.netlify.com

[codecov]

Codecov Report

Merging #3284 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3284   +/-   ##
=======================================
  Coverage   97.38%   97.38%           
=======================================
  Files         559      559           
  Lines       11648    11648           
  Branches      837      837           
=======================================
  Hits        11343    11343           
  Misses        194      194           
  Partials      111      111

---

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 e4c50c3...f42f13a. Read the comment docs.