修正可空枚举ToJson

ModelTypes/ComplexModelType.cs

@@ -25,6 +25,10 @@ namespace FlutterCodeGenerator.ModelTypes
                 {
                     source.AppendLine($"\t\tmap['{child.Name_Upper}'] = {child.Name_Lower}.index;");
                 }
+                else if (child is EnumNullableModelType)
+                {
+                    source.AppendLine($"\t\tmap['{child.Name_Upper}'] = {child.Name_Lower}?.index;");
+                }
                 else if (child.DefaultValue != null)
                 {
                     source.AppendLine($"\t\tmap['{child.Name_Upper}'] = {child.Name_Lower};");

ModelTypes/UserDefinedDerivedModelType.cs

@@ -187,6 +187,10 @@ namespace FlutterCodeGenerator.ModelTypes
                         {
                             source.AppendLine($"\t\tmap['{child.Name_Upper}'] = {child.Name_Lower}.index;");
                         }
+                        else if (child is EnumNullableModelType)
+                        {
+                            source.AppendLine($"\t\tmap['{child.Name_Upper}'] = {child.Name_Lower}?.index;");
+                        }
                         else if (child.DefaultValue != null)
                         {
                             source.AppendLine($"\t\tmap['{child.Name_Upper}'] = {child.Name_Lower};");