entc/gen: add feature-flag to set unique backref when their inverse is loaded (#3953)

examples/start/ent/car.go

@@ -45,12 +45,10 @@ type CarEdges struct {
 // OwnerOrErr returns the Owner value or an error if the edge
 // was not loaded in eager-loading, or loaded but was not found.
 func (e CarEdges) OwnerOrErr() (*User, error) {
-	if e.loadedTypes[0] {
-		if e.Owner == nil {
-			// Edge was loaded but was not found.
-			return nil, &NotFoundError{label: user.Label}
-		}
+	if e.Owner != nil {
 		return e.Owner, nil
+	} else if e.loadedTypes[0] {
+		return nil, &NotFoundError{label: user.Label}
 	}
 	return nil, &NotLoadedError{edge: "owner"}
 }