Fix type definition bug

[Lazyuki]

I'm getting this error.

node_modules/rc-slider/lib/Slider.d.ts:3:18 - error TS2430: Interface 'SliderProps' incorrectly extends interface 'GenericSliderProps'.
  Types of property 'step' are incompatible.
    Type 'number | null | undefined' is not assignable to type 'number | undefined'.
      Type 'null' is not assignable to type 'number | undefined'.

3 export interface SliderProps extends GenericSliderProps {
                   ~~~~~~~~~~~

This is breaking my build so it would be nice if you could release a new patch version after this PR. Thanks.

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[codecov]

Codecov Report

Merging #695 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #695   +/-   ##
=======================================
  Coverage   88.26%   88.26%           
=======================================
  Files          10       10           
  Lines         690      690           
  Branches      181      181           
=======================================
  Hits          609      609           
  Misses         81       81           

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