[ASLayout] Small optimization to eliminate one method call in an extremely hot codepath.

This was observed with quantitative profiling results.  We have some opportunities to micro-optimize the layout objects.

AsyncDisplayKit/Layout/ASLayout.mm

@@ -113,6 +113,7 @@ extern BOOL CGPointIsNull(CGPoint point)
   return [self layoutWithLayoutableObject:layoutableObject
                      constrainedSizeRange:sizeRange
                                      size:size
+                                 position:CGPointNull
                                sublayouts:nil];
 }