Add unit test for ImageView to take null uri in source

Summary:
This is a follow up action item from S295231 and T136039462 where we want to make sure null uri in image source is handled properly. This diff adds an unit test to make sure we are using transparent image when uri is null.

Changelog:
[Android][Internal] - Add unit test to ImageView for null uri in source

Reviewed By: javache

Differential Revision: D40732791

fbshipit-source-id: fd468bfe7c33a4f3f8913ead3e84a1770d7c907f

ReactAndroid/src/main/java/com/facebook/react/views/image/ReactImageView.java

@@ -540,6 +540,11 @@ public class ReactImageView extends GenericDraweeView {
     maybeUpdateView();
   }
 
+  // VisibleForTesting
+  public @Nullable ImageSource getImageSource() {
+    return mImageSource;
+  }
+
   @Override
   protected void onSizeChanged(int w, int h, int oldw, int oldh) {
     super.onSizeChanged(w, h, oldw, oldh);