dont use ios border rendering if we use default black color

Summary:
iOS and css have a different way to draw borders. Decision which rendering to use considers alpha value of given color as well.
If alpha=0 -> border is invisible -> we use ios render.
That doesn't work as expected in a case where only `borderWidth` property is set.
A default black color is used when no `borderColor` is set, but alpha checking logic will test `null`. Which resolves as alpha=0.

Reviewed By: javache

Differential Revision: D3750523

fbshipit-source-id: 453d52a16287c6286e8887a9ab3795d3ded1dec6

React/Views/RCTView.m

@@ -477,7 +477,7 @@ - (void)displayLayer:(CALayer *)layer
   // the content. For this reason, only use iOS border drawing when clipping
   // or when the border is hidden.
 
-  (borderInsets.top == 0 || CGColorGetAlpha(borderColors.top) == 0 || self.clipsToBounds);
+  (borderInsets.top == 0 || (borderColors.top && CGColorGetAlpha(borderColors.top) == 0) || self.clipsToBounds);
 
   // iOS clips to the outside of the border, but CSS clips to the inside. To
   // solve this, we'll need to add a container view inside the main view to