Fix UNDEFINED YGValue serialization (#52376)

Summary:
Pull Request resolved: #52376

Correct the expected serialization based on the YogaValue parse implementation:
https://www.internalfb.com/code/fbsource/[ecdf90fe69d0]/xplat/js/react-native-github/packages/react-native/ReactAndroid/src/main/java/com/facebook/yoga/YogaValue.java?lines=66-68

Changelog: [Internal]

Reviewed By: mdvacca

Differential Revision: D77663413

fbshipit-source-id: e8b6091a5d57c6d0301411371c290a867c9b5224

Author: Nick Lefever

packages/react-native/ReactCommon/react/renderer/core/graphicsConversions.h

@@ -65,7 +65,7 @@ inline std::string toString(const SharedColor& value) {
 inline folly::dynamic toDynamic(const YGValue& dimension) {
   switch (dimension.unit) {
     case YGUnitUndefined:
-      return nullptr;
+      return "undefined";
     case YGUnitAuto:
       return "auto";
     case YGUnitMaxContent: