Tests for enum type conversion from mirror

graphql_schema/lib/src/enum.dart

@@ -24,6 +24,7 @@ class GraphQLEnumType<Value> extends GraphQLScalarType<Value, String>
 
   @override
   String serialize(Value value) {
+    if (value == null) return null;
     return values.firstWhere((v) => v.value == value).name;
   }
 

graphql_server/test/mirrors_test.dart

@@ -28,6 +28,10 @@ void main() {
         expect(asEnumType.serialize(RomanceLanguage.FRANCE), 'FRANCE');
       });
 
+      test('can serialize null', () {
+        expect(asEnumType.serialize(null), null);
+      });
+
       test('fails to serialize invalid value', () {
         expect(() => asEnumType.serialize(34), throwsStateError);
       });