[FIX] mail: abstract getAvatarSource has default

Steps to reproduce:
- install sales
- create a new quotation

Previous behavior:
the avatar next to the author's name is empty in the mailthread

Current behavior:
avatar is displayed as expected

opw-2187231

closes odoo/odoo#44379

Signed-off-by: mightyjol <jhk-odoo@users.noreply.github.com>

addons/mail/static/src/js/models/messages/abstract_message.js

@@ -92,7 +92,11 @@ var AbstractMessage =  Class.extend({
      * @abstract
      * @return {string}
      */
-    getAvatarSource: function () {},
+    getAvatarSource: function () {
+        if (this.hasAuthor()) {
+            return '/web/image/res.partner/' + this.getAuthorID() + '/image_128';
+        }
+    },
     /**
      * Get the body content of this message
      *