Improving Modal visible prop check to handle undefined and null (#22072)

Summary:
Changing the the this.props.visible if to be ` if (!!this.props.visible === false)` . So passing undefined, or other values wont set the modal to be visible. Granting that anything that is not true, will set the modal to null on the render.

I make this this PR, because on the company that im working, we used a lot of RN. At the moment, we arent using anything like flow or TS. But to grant that the modals will only show if they are set visible to true i have changed the if made.

_Pull requests that expand test coverage are more likely to get reviewed. Add a test case whenever possible!_
Pull Request resolved: https://github.com/facebook/react-native/pull/22072

Differential Revision: D12918086

Pulled By: TheSavior

fbshipit-source-id: 8f9c63ac6fd56d83949bb8428ad7c5b7bf805c49

Libraries/Modal/Modal.js

@@ -201,7 +201,7 @@ class Modal extends React.Component<Object> {
   }
 
   render(): React.Node {
-    if (this.props.visible === false) {
+    if (this.props.visible !== true) {
       return null;
     }