fix: page crash when dots is true and step is null

[denghuacc]

resolve ant-design/ant-design#36387

[vercel]

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Name	Status	Preview	Updated
slider	✅ Ready (Inspect)	Visit Preview	Jul 6, 2022 at 7:39AM (UTC)

[MadCcc]

Could you provide a test case?

[denghuacc]

有没有更好的测试方法？
在没有修复这个 Bug 之前，如果使用了这个测试用例是会一直卡住的，无法结束测试。

[codecov]

Codecov Report

Merging #838 (ad819ad) into master (b50dbed) will not change coverage.
The diff coverage is 100.00%.

❗ Current head ad819ad differs from pull request most recent head e8fe468. Consider uploading reports for the commit e8fe468 to get more accurate results

@@           Coverage Diff           @@
##           master     #838   +/-   ##
=======================================
  Coverage   99.81%   99.81%           
=======================================
  Files          13       13           
  Lines         535      535           
  Branches      148      148           
=======================================
  Hits          534      534           
  Misses          1        1           

Impacted Files	Coverage Δ
src/Steps/index.tsx	100.00% <100.00%> (ø)

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