[FIX] digest: avoid exception on user creation

Scenario to reproduce the issue on runbot 12.0 Community:
- Go to Settings > Technical > Email > Digest Emails
- Select the Weekly Digest
- Click on Action > Delete
- Click on OK
- Go to Settings > Users & Companies > Users
- Click on Create
- Fill the Name and the Email Address
- Save
- Odoo Server Error - Missing Record

Some users seem to delete this record to stop receiving the digest for
everyone, including for future users.  The problem is that, even if the
digest has been deleted, the config parameters are still referencing it.

This commit prevents the exception by having an empty recordset if the
digest does not exist.

closes odoo/odoo#73077

Signed-off-by: Paul Morelle <madprog@users.noreply.github.com>

addons/digest/models/res_users.py

@@ -13,6 +13,7 @@ class ResUsers(models.Model):
         default_digest_emails = self.env['ir.config_parameter'].sudo().get_param('digest.default_digest_emails')
         default_digest_id = self.env['ir.config_parameter'].sudo().get_param('digest.default_digest_id')
         if user.has_group('base.group_user') and default_digest_emails and default_digest_id:
-            digest = self.env['digest.digest'].sudo().browse(int(default_digest_id))
-            digest.user_ids |= user
+            digest = self.env['digest.digest'].sudo().browse(int(default_digest_id)).exists()
+            if digest:
+                digest.user_ids |= user
         return user