@types/react: Update React.Component setState

Allow setState to accept an updater which is null or is a function that
may return null.

In React 16, passing null to setState or returning null from the updater
function does not cause an update.

types/react/index.d.ts

@@ -284,7 +284,7 @@ declare namespace React {
         // See: https://github.com/DefinitelyTyped/DefinitelyTyped/issues/18365#issuecomment-351013257
         // Also, the ` | S` allows intellisense to not be dumbisense
         setState<K extends keyof S>(
-            state: ((prevState: Readonly<S>, props: P) => (Pick<S, K> | S)) | (Pick<S, K> | S),
+            state: ((prevState: Readonly<S>, props: P) => (Pick<S, K> | S | null)) | (Pick<S, K> | S | null),
             callback?: () => void
         ): void;
 

types/react/test/tsx.tsx

@@ -95,6 +95,7 @@ class SetStateTest extends React.Component<{}, { foo: boolean, bar: boolean }> {
       this.setState({ foo: true });
       this.setState({ foo: true, bar: true });
       this.setState({});
+      this.setState(null);
       this.setState({ foo: true, foo2: true }); // $ExpectError
       this.setState(() => ({ foo: '' })); // $ExpectError
       this.setState(() => ({ foo: true }));
@@ -103,6 +104,7 @@ class SetStateTest extends React.Component<{}, { foo: boolean, bar: boolean }> {
       this.setState(() => ({ foo: '', foo2: true })); // $ExpectError
       this.setState(() => ({ })); // ok!
       this.setState({ foo: true, bar: undefined}); // $ExpectError
+      this.setState(prevState => (prevState.bar ? { bar: false } : null));
     }
 }