db: prevent empty IN queries that generally are performance problem, and triggers sql warnings.

rhodecode/apps/admin/views/user_groups.py

@@ -100,7 +100,7 b' class AdminUserGroupsView(BaseAppView, D'
         auth_user_group_list = UserGroupList(
             UserGroup.query().all(), perm_set=['usergroup.admin'])
 
-        allowed_ids = []
+        allowed_ids = [-1]
         for user_group in auth_user_group_list:
                 allowed_ids.append(user_group.users_group_id)
 

rhodecode/apps/repository/views/repo_forks.py

@@ -90,7 +90,7 b' class RepoForksView(RepoAppView, DataGri'
         acl_check = HasRepoPermissionAny(
             'repository.read', 'repository.write', 'repository.admin')
         repo_id = self.db_repo.repo_id
-        allowed_ids = []
+        allowed_ids = [-1]
         for f in Repository.query().filter(Repository.fork_id == repo_id):
             if acl_check(f.repo_name, 'get forks check'):
                 allowed_ids.append(f.repo_id)