Typings: correct typings to match propTypes

[layershifter]

Bug Report

The latest release of @types/react with TS 3.2 breaks all our builds on Project Tests stage.

Steps

Build any PR.

Expected Result

CI pass.

Actual Result

[layershifter]

DefinitelyTyped/DefinitelyTyped#30730 (comment)

[layershifter]

An example of failed build #549.

[layershifter]

It's not clear to me what will be done with these changes in @types/react.

I think that for now the best option for us will be use yarn's resolutions to get the stable release of @types/react.

[layershifter]

DefinitelyTyped/DefinitelyTyped#30730 (comment)
https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/prop-types/index.d.ts#L40

The problem is with types difference.

[layershifter]

Proposed solution:

type Optional<T> = T | null | undefined 

-interface CProps {
-  required?: boolean
-  type?: string
-}
+interface CProps {
+  required: Optional<boolean>
+  type: Optional<string>
+}

[layershifter]

https://github.com/facebook/prop-types/blob/v15.6.2/factoryWithTypeCheckers.js#L205

[layershifter]

We decided to go with:

type Nullable<T> = T | null

-interface CProps {
-  required?: boolean
-  type?: string
-}
+interface CProps {
+  required: Nullable<boolean>
+  type: Nullable<string>
+}