[FIX] mail: alias: default value for alias_domain

otherwise it is not correctly computed on new records inheriting
from mail.alias class.

addons/mail/models/mail_alias.py

@@ -62,7 +62,8 @@ class Alias(models.Model):
         'Record Thread ID',
         help="Optional ID of a thread (record) to which all incoming messages will be attached, even "
              "if they did not reply to it. If set, this will disable the creation of new records completely.")
-    alias_domain = fields.Char('Alias domain', compute='_get_alias_domain')
+    alias_domain = fields.Char('Alias domain', compute='_get_alias_domain',
+                               default=lambda self: self.env["ir.config_parameter"].get_param("mail.catchall.domain"))
     alias_parent_model_id = fields.Many2one(
         'ir.model', 'Parent Model',
         help="Parent model holding the alias. The model holding the alias reference"