fix: handle null only for outer list

execution/engine/execution_engine_grpc_test.go

@@ -1432,4 +1432,23 @@ func TestGRPCSubgraphExecution(t *testing.T) {
 		require.Contains(t, response, `"teamsByProject":`)
 		require.Contains(t, response, `"favoriteCategories":`)
 	})
+
+	t.Run("should handle empty and nullable list items", func(t *testing.T) {
+		operation := graphql.Request{
+			OperationName: "EmptyAndNullableListItems",
+			Query: `query EmptyAndNullableListItems {
+				author {
+					id
+					authorGroups {
+						id
+						name
+					}
+				}
+			}`,
+		}
+
+		response, err := executeOperation(t, conn, operation, withGRPCMapping(mapping.DefaultGRPCMapping()))
+		require.NoError(t, err)
+		require.Equal(t, `{"data":{"author":{"id":"author-default","authorGroups":[[{"id":"group-auth-1","name":"Team Lead Alpha"},{"id":"group-auth-2","name":"Senior Dev Beta"}],[{"id":"group-auth-3","name":"Junior Dev Gamma"}],[],null]}}}`, response)
+	})
 }

v2/pkg/engine/datasource/grpc_datasource/grpc_datasource.go

@@ -299,11 +299,12 @@ func (d *DataSource) traverseList(level int, arena *astjson.Arena, current *astj
 
 	msg := data.Get(fd).Message()
 	if !msg.IsValid() {
+		// If the message is not valid we can either return null if the list is nullable or an error if it is non nullable.
 		if md.LevelInfo[level].Optional {
 			return arena.NewNull(), nil
 		}
 
-		return arena.NewArray(), nil
+		return arena.NewArray(), fmt.Errorf("cannot add null item to response for non nullable list")
 	}
 
 	fd = msg.Descriptor().Fields().ByNumber(1)
@@ -328,11 +329,9 @@ func (d *DataSource) traverseList(level int, arena *astjson.Arena, current *astj
 
 	list := msg.Get(fd).List()
 	if !list.IsValid() {
-		if md.LevelInfo[level].Optional {
-			return arena.NewNull(), nil
-		}
-
-		return arena.NewNull(), fmt.Errorf("cannot add null item to response for non nullable list")
+		// If the list is not valid, we return an empty array here as the
+		// nullabilty is checked on the outer List wrapper type.
+		return arena.NewArray(), nil
 	}
 
 	for i := 0; i < list.Len(); i++ {

v2/pkg/grpctest/mockservice.go

@@ -1628,6 +1628,10 @@ func (s *MockService) QueryAuthor(ctx context.Context, in *productv1.QueryAuthor
 							{Id: "group-auth-3", Name: "Junior Dev Gamma"},
 						},
 					}},
+					// empty list
+					{List: &productv1.ListOfUser_List{}},
+					// null item
+					nil,
 				},
 			},
 		},