fix: allow step prop to be null for typescript

[shangyilim]

When using null step Slider in Typescript, the linter complains that step cannot be null when used:

export default () => (
  <div>
       {/*Type 'null' is not assignable to type 'number | undefined'*/}
       <Slider step={null} /> 
    </div>
)

In the API is is clearly mentioned that step can be null:

Name	Type	Default	Description
step	number or null	1	Value to be added or subtracted on each step the slider makes. Must be greater than zero, and max - min should be evenly divisible by the step value. When marks is not an empty object, step can be set to null, to make marks as steps.

This PR is to fix the type definition so that it doesn't show the error above.

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[codecov]

Codecov Report

Merging #691 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #691   +/-   ##
=======================================
  Coverage   88.11%   88.11%           
=======================================
  Files          10       10           
  Lines         690      690           
  Branches      182      182           
=======================================
  Hits          608      608           
  Misses         82       82           

Impacted Files	Coverage Δ
src/Slider.tsx	92.22% <ø> (ø)

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