[FIX] project: fix groupby by none issue in project sharing

Steps:
- Go to portal and select tasks and search anything in search bar.
- Showing  no tasks available
- choose group by none and search anything it will show table labels.

Fix:
shows warning when there is no record when groupby by none.

task-3183771

closes odoo/odoo#114406

Signed-off-by: Xavier Bol (xbo) <xbo@odoo.com>

addons/project/controllers/portal.py

@@ -201,7 +201,7 @@ class CustomerPortal(CustomerPortal):
         elif groupby == 'stage':
             grouped_tasks = [request.env['project.task'].concat(*g) for k, g in groupbyelem(tasks, itemgetter('stage_id'))]
         else:
-            grouped_tasks = [tasks]
+            grouped_tasks = [tasks] if tasks else []
 
         values.update({
             'date': date_begin,