[FIX] event_sale: event tickets domain in sale order line

From saas-6, setting `0` as maximum available seats
means there is an unlimited amount of tickts that
can be sold, as stated in the help message of the field:
```
Define the number of available tickets. If you have too much registrations you will
not BE able to sell tickets anymore. Set 0 to ignore this rule set as unlimited.
```

This domain is the same domain used than in
the QWeb views of `website_event_sale`.

opw-669355

addons/event_sale/views/sale_order.xml

@@ -8,7 +8,7 @@
              <field name="arch" type="xml">
                 <xpath expr="//field[@name='product_id']" position="after">
                     <field name="event_id" domain="['|', ('type','=', False),('type', '=', event_type_id),('date_end','&gt;=',time.strftime('%Y-%m-%d 00:00:00'))]" attrs="{'invisible': [('event_ok', '=', False)],'required': [('event_ok', '!=', False)]}"/>
-                    <field name="event_ticket_id" domain="[('event_id', '=', event_id), ('seats_available', '>', 0)]" attrs="{'invisible': [('event_id', '=', False)]}"
+                    <field name="event_ticket_id" domain="[('event_id', '=', event_id), '|', ('seats_available', '>', 0), ('seats_availability', '=', 'unlimited')]" attrs="{'invisible': [('event_id', '=', False)]}"
                         on_change="onchange_event_ticket_id(event_ticket_id, context)"/>
                     <field name="event_type_id" invisible="1"/>
                     <field name="event_ok" invisible="1"/>