Allow passing null to onNavigationStateChange prop (react-navigation#3683)

As seen here: react-navigation#360, we need to be able to pass null to turn off logging.

Author: benadamstyles

flow/react-navigation.js

@@ -519,7 +519,7 @@ declare module 'react-navigation' {
 
   declare export type NavigationContainerProps<S: {}, O: {}> = $Shape<{
     uriPrefix?: string | RegExp,
-    onNavigationStateChange?: (
+    onNavigationStateChange?: ?(
       NavigationState,
       NavigationState,
       NavigationAction