entc/gen: change eager-loading empty values (#791)

When loading relations (using edger-loading) and there are no changes,
the response should be an empty slice (and not nil). Similar to the
way we handle simple queries (e.g. All()).

Closed #790

examples/start/ent/group_query.go

@@ -369,6 +369,7 @@ func (gq *GroupQuery) sqlAll(ctx context.Context) ([]*Group, error) {
 		for _, node := range nodes {
 			ids[node.ID] = node
 			fks = append(fks, node.ID)
+			node.Edges.Users = []*User{}
 		}
 		var (
 			edgeids []int

examples/start/ent/user_query.go

@@ -405,6 +405,7 @@ func (uq *UserQuery) sqlAll(ctx context.Context) ([]*User, error) {
 		for i := range nodes {
 			fks = append(fks, nodes[i].ID)
 			nodeids[nodes[i].ID] = nodes[i]
+			nodes[i].Edges.Cars = []*Car{}
 		}
 		query.withFKs = true
 		query.Where(predicate.Car(func(s *sql.Selector) {
@@ -433,6 +434,7 @@ func (uq *UserQuery) sqlAll(ctx context.Context) ([]*User, error) {
 		for _, node := range nodes {
 			ids[node.ID] = node
 			fks = append(fks, node.ID)
+			node.Edges.Groups = []*Group{}
 		}
 		var (
 			edgeids []int