Fix jest test crashes with animated components

Summary:
In the jest test renderer, host components have null refs by default. `createAnimatedComponent` tries to access the ref in componentDidMount, which then crashes. This is particularly problematic when trying to update test data:

https://fb.workplace.com/groups/mpen00bs/permalink/494236684721027/?comment_id=510656413079054

Just checking for null fixes the issue and shouldn't affect anything else.

Reviewed By: TheSavior, yungsters

Differential Revision: D16777137

fbshipit-source-id: 0b9f7c5734c849f36318512ceffcc42dd44c58bb

Libraries/Animated/src/createAnimatedComponent.js

@@ -63,7 +63,7 @@ function createAnimatedComponent(Component: any, defaultProps: any): any {
     _attachNativeEvents() {
       // Make sure to get the scrollable node for components that implement
       // `ScrollResponder.Mixin`.
-      const scrollableNode = this._component.getScrollableNode
+      const scrollableNode = this._component?.getScrollableNode
         ? this._component.getScrollableNode()
         : this._component;