[FIX] calendar: allow to schedule meetings/calls when already created

When a an activity was already created, it is no longer to schedule it
when we change the type to a call or a meeting.

Step to reproduce the issue:
1. Create any activity (without scheduling it) on for example a Quotation
2. Edit this activity and change its type to meeting/call
There are no longer a Open Calendar Button, thus we can't schedule it.

Solution: The issue is simply that the Open Calendar button is only displayed
when the activity is not yet created (and if we have the appropriate category).
Instead of this condition, we should display this button if there are no event
already created.

opw-2824396

closes odoo/odoo#90028

X-original-commit: 1a66a3c4478c5b98951b658b6efbbd0b0d82166d
Signed-off-by: Anh Thao PHAM <pta@odoo.com>
Signed-off-by: Desausoi Laurent (lade) <lade@odoo.com>

addons/calendar/views/mail_activity_views.xml

@@ -25,11 +25,12 @@
                   <attribute name="attrs">{'invisible': [('activity_category', '=', 'meeting')]}</attribute>
             </xpath>
             <xpath expr="//button[@name='action_close_dialog']" position="before">
-                <button string="Open Calendar"
-                    attrs="{'invisible': ['|', ('activity_category', 'not in', ['meeting', 'phonecall']), ('id', '!=', False)]}"
-                    name="action_create_calendar_event"
-                    type="object"
-                    class="btn-primary"/>
+                  <field name="calendar_event_id" invisible="1" />
+                  <button string="Open Calendar"
+                        attrs="{'invisible': ['|', ('activity_category', 'not in', ['meeting', 'phonecall']), ('calendar_event_id', '!=', False)]}"
+                        name="action_create_calendar_event"
+                        type="object"
+                        class="btn-primary"/>
             </xpath>
         </field>
     </record>