Don't return empty string for accessibilityLabel for RCTView

Summary:
In case `RCTView` had no subviews, `RCTRecursiveAccessibilityLabel` was returning an empty string rather than nil.

I've noticed that https://fburl.com/i95iynxi returns empty string as well. Shall we change it there as well?

Reviewed By: shergin

Differential Revision: D15337865

fbshipit-source-id: 8e38a88321f3dcfdf35a38d43fdbe4f5118796f1

React/Views/RCTView.m

@@ -95,7 +95,7 @@ - (UIView *)react_findClipView
       [str appendString:label];
     }
   }
-  return str;
+  return str.length == 0 ? nil : str;
 }
 
 @implementation RCTView