Avoid serializing UndefinedColor when rendering texts

Summary: This diff avoids the serialization of color that are set as: "UndefinedColor". This allows the text rendering system to set the default color in the native side

Reviewed By: shergin

Differential Revision: D10275834

fbshipit-source-id: b81c7a5995bef65e04a246d99f44ff10cb20f548

ReactCommon/fabric/attributedstring/conversions.h

@@ -387,7 +387,9 @@ inline folly::dynamic toDynamic(
 
 inline folly::dynamic toDynamic(const TextAttributes &textAttributes) {
   auto _textAttributes = folly::dynamic::object();
+  if (textAttributes.foregroundColor) {
     _textAttributes("foregroundColor", toDynamic(textAttributes.foregroundColor));
+  }
   if (textAttributes.backgroundColor) {
     _textAttributes(
         "backgroundColor", toDynamic(textAttributes.backgroundColor));