Add convert compatible of NSString for bridge message data (#24630)

Summary:
Fixes https://twitter.com/estevao_lucas/status/1117572702083190785?s=215 in #24626 . Now we try to convert any id to `NSString`, not throw error.

cc. cpojer .

[iOS] [Fixed] - Add convert compatible of NSString for bridge message data
Pull Request resolved: #24630

Differential Revision: D15120205

Pulled By: cpojer

fbshipit-source-id: 4849a8e941410b292f971608a9cdb38c11502445

React/Base/RCTConvert.m

@@ -53,12 +53,21 @@ + (type *)type:(id)json { return json; }
 
 RCT_JSON_CONVERTER(NSArray)
 RCT_JSON_CONVERTER(NSDictionary)
-RCT_JSON_CONVERTER(NSString)
 RCT_JSON_CONVERTER(NSNumber)
 
 RCT_CUSTOM_CONVERTER(NSSet *, NSSet, [NSSet setWithArray:json])
 RCT_CUSTOM_CONVERTER(NSData *, NSData, [json dataUsingEncoding:NSUTF8StringEncoding])
 
++ (NSString *)NSString:(id)json
+{
+  if ([json isKindOfClass:NSString.class]) {
+    return json;
+  } else if (json && json != (id)kCFNull) {
+    return [NSString stringWithFormat:@"%@",json];
+  }
+  return nil;
+}
+
 + (NSIndexSet *)NSIndexSet:(id)json
 {
   json = [self NSNumberArray:json];