[FIX] event: do not erase existing tags when changing category

PURPOSE

Prepare Event Online support by providing fixes and preparatory cleaning
commits.

SPECIFICATIONS

If an event has tags, changing its category resets them. We consider that
a category should update an event but not force or reset event tags, only
give default values.

LINKS

Task ID 2289148 (Event Bugprovements)
Prepares Task ID 2252655 (master Online Event task)
Prepares Task ID 2283796 (Event B2Basics)
PR #53944

X-original-commit: 408bad96de293dc10a3aa78fcee6159d8012fadc

addons/event/models/event_event.py

@@ -376,7 +376,8 @@ class EventEvent(models.Model):
                 event.seats_limited = event.event_type_id.has_seats_limitation
 
             event.auto_confirm = event.event_type_id.auto_confirm
-            event.tag_ids = event.event_type_id.tag_ids
+            if not event.tag_ids and event.event_type_id.tag_ids:
+                event.tag_ids = event.event_type_id.tag_ids
 
     @api.depends('event_type_id')
     def _compute_event_mail_ids(self):