@types/react-dates: Add null as valid value for FocusedInputShape

React dates accepts but more importantly returns `null` as a valid focused state.
Not having this in the typings allows unexpected errors when using `strictNulls` checking.
```typescript
if (focusedInput !== undefined) {
  // focusedInput input could be null here, but the type system does not acknowledge that. 
}
```

react-dates/index.d.ts

@@ -18,7 +18,7 @@ declare namespace momentPropTypes{
     
 declare namespace ReactDates{
     type AnchorDirectionShape = 'left' | 'right';
-    type FocusedInputShape = 'startDate' | 'endDate';    
+    type FocusedInputShape = 'startDate' | 'endDate' | null;
     type OrientationShape = 'horizontal' | 'vertical';        
     type ScrollableOrientationShape = 'horizontal' | 'vertical' | 'verticalScrollable';