Type of `ValueType` is incorrect - is `string | number | null`, should be `string | number`

[fnimick]

null was added to resolve ant-design/ant-design#37412 - however, the fix in #470 was incorrect. Having null as a valid value for all ValueType entries breaks compatibility with React.InputHTMLAttributes min, max, and step properties which explicitly exclude null. This is preventing this library from being used in projects with strict null checks enabled.

The correct solution here is to add null to the input value type of the onChange function, and nowhere else. It is explicitly called with null only in one special case as outlined in https://github.com/react-component/input-number/blob/master/src/InputNumber.tsx#L313