[FIX] mail: get systray activities for custom models

Getting systray activities for custom models (i.e. created with Studio) crashed.

If it's a custom model, its `_original_module` attribute is not correctly set so
trying to retrieve the icon based on this attribute does not work.

addons/mail/models/res_users.py

@@ -129,11 +129,13 @@ GROUP BY channel_moderator.res_users_id""", [tuple(self.ids)])
         user_activities = {}
         for activity in activity_data:
             if not user_activities.get(activity['model']):
+                module = self.env[activity['model']]._original_module
+                icon = module and modules.module.get_module_icon(module)
                 user_activities[activity['model']] = {
                     'name': model_names[activity['id']],
                     'model': activity['model'],
                     'type': 'activity',
-                    'icon': modules.module.get_module_icon(self.env[activity['model']]._original_module),
+                    'icon': icon,
                     'total_count': 0, 'today_count': 0, 'overdue_count': 0, 'planned_count': 0,
                 }
             user_activities[activity['model']]['%s_count' % activity['states']] += activity['count']

addons/mail/static/src/xml/systray.xml

@@ -70,7 +70,7 @@
         </t>
         <t t-foreach="activities" t-as="activity">
             <div class="o_mail_preview" t-att-data-res_model="activity.model" t-att-data-model_name="activity.name" data-filter='my'>
-                <div class="o_mail_preview_image o_mail_preview_app">
+                <div t-if="activity.icon" class="o_mail_preview_image o_mail_preview_app">
                     <img t-att-src="activity.icon" alt="Activity"/>
                 </div>
                 <div class="o_preview_info">