[IMP] mail: improve default channel picture

Purpose is to avoid having void image in systray when there are notifications
linked to channels that have no picture. Two small improvements are done on
channel model
 * change default picture for channel to be more inline with Odoo design
   guidelines;
 * override create to ensure there is a default image when creating the
   channel through the discuss interface;

This commit is linked to task ID 48228.

addons/mail/models/mail_channel.py

@@ -115,6 +115,11 @@ class Channel(models.Model):
 
     @api.model
     def create(self, vals):
+        # ensure image at quick create
+        if not vals.get('image'):
+            defaults = self.default_get(['image'])
+            vals['image'] = defaults['image']
+
         tools.image_resize_images(vals)
         # Create channel and alias
         channel = super(Channel, self.with_context(