fix mostly bugs on system management, and user roles and group assignment

backend/internal/iam/group.go

@@ -88,11 +88,14 @@ func GetUserGroups(db *database.DB, userID string) ([]string, error) {
 	for rows.Next() {
 		var groupName string
 		if err := rows.Scan(&groupName); err != nil {
-			return nil, err
+			return []string{}, err
 		}
 		groups = append(groups, groupName)
 	}
 
+	if groups == nil {
+		groups = []string{}
+	}
 	return groups, rows.Err()
 }