nil instead of empty `RCTViewManagerUIBlock` block

Summary: There is no point in returning empty block here. It's a tiny bit wasteful and prevents us from knowing if there are any UI updates.

Depends on D2571122 (not really).

public

Reviewed By: jspahrsummers

Differential Revision: D2571137

fb-gh-sync-id: 4a872f33f05eaf2d858009aa04ea89feaa23576c

Libraries/Text/RCTTextManager.m

@@ -88,7 +88,7 @@ RCT_EXPORT_SHADOW_PROPERTY(allowFontScaling, BOOL)
     }
   }
 
-  return ^(RCTUIManager *uiManager, RCTSparseArray *viewRegistry) {};
+  return nil;
 }
 
 - (RCTViewManagerUIBlock)uiBlockToAmendWithShadowView:(RCTShadowText *)shadowView