Clean up FieldValue port (#8435)

Author: schmidt-sebastian

Firestore/Source/API/FSTUserDataReader.mm

@@ -305,8 +305,7 @@ - (ParsedUpdateData)parsedUpdateData:(id)input {
                                               context:(ParseContext &&)context {
   __block Message<google_firestore_v1_Value> result;
   result->which_value_type = google_firestore_v1_Value_map_value_tag;
-  result->map_value.fields_count = 0;
-  result->map_value.fields = nil;
+  result->map_value = {};
 
   if (dict.count == 0) {
     const FieldPath *path = context.path();