Fix: text shadow displays on iOS when textShadowOffset is {0,0} (#24398)

Summary:
There is a problem rendering text shadows on iOS. If the offset of the text shadow is `{width:0,height:0}`, the shadow does not display. This prevents you from representing a light directly above the text. This occurs because a text shadow only renders if the offset is a non-zero CGRect `{width:0,height:0}`.

My change checks `textShadowRadius` instead. If `textShadowRadius` is not nan then the user is rendering a text shadow. There are no situations to render a shadow without `textShadowRadius` making it a good variable to check.

This PR fixes this stale issue: #17277

[iOS] [Fixed] - Text shadow now displays when the textShadowOffset is {width:0,height:0}
Pull Request resolved: #24398

Differential Revision: D14890768

Pulled By: cpojer

fbshipit-source-id: a43b96a4a04a5603eede466abacd95c010d053e5

Libraries/Text/RCTTextAttributes.m

@@ -170,7 +170,7 @@ - (NSParagraphStyle *)effectiveParagraphStyle
   }
 
   // Shadow
-  if (!CGSizeEqualToSize(_textShadowOffset, CGSizeZero)) {
+  if (!isnan(_textShadowRadius)) {
     NSShadow *shadow = [NSShadow new];
     shadow.shadowOffset = _textShadowOffset;
     shadow.shadowBlurRadius = _textShadowRadius;