fix: dropdown button disable style

[skyline-123]

First of all, thank you for your contribution! 😄

New feature please send pull request to feature branch, and rest to master branch.
Pull request will be merged after one of collaborators approve.
Please makes sure that these form are filled before submitting your pull request, thank you!

[中文版模板 / Chinese template]

This is a ...

• New feature
• Bug fix
• Site / document update
• Component style update
• TypeScript definition update
• Refactoring
• Code style optimization
• Branch merge
• Other (about what?)

What's the background?

1. Describe the source of requirement.
   I found the right button of the drop-down menu can still be clicked on by comparing it with the ant-react.
2. Resolve what problem.
   Fixed disabled state of dropdown menu with button.
3. Related issue link.

API Realization (Optional if not new feature)

1. Basic thought of solution and other optional proposal.
2. List final API realization and usage sample.
3. GIF or snapshot should be provided if includes UI/interactive modification.

What's the effect? (Optional if not new feature)

1. Does this PR affect user? Which part will be affected?
2. What will say in changelog?
3. Does this PR contains potential break change or other risk?

Changelog description (Optional if not new feature)

1. English description
2. Chinese description (optional)

Self Check before Merge

• Doc is updated/provided or not needed
• Demo is updated/provided or not needed
• TypeScript definition is updated/provided or not needed
• Changelog is provided or not needed

Additional Plan? (Optional if not new feature)

If this PR related with other PR or following info. You can type here.

[netlify]

Deploy preview for ant-desing-vue processing.

Building with commit f6615e6

https://app.netlify.com/sites/ant-desing-vue/deploys/5d14f702f179f100090e8666

[codecov]

Codecov Report

Merging #912 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #912   +/-   ##
=======================================
  Coverage   87.27%   87.27%           
=======================================
  Files         145      145           
  Lines        4817     4817           
  Branches     1421     1421           
=======================================
  Hits         4204     4204           
  Misses        570      570           
  Partials       43       43

Impacted Files	Coverage Δ
components/dropdown/dropdown-button.jsx	86.66% <ø> (ø)	⬆️

---

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 d0cf9fd...f6615e6. Read the comment docs.

[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.