[FIX] mail_enterprise: display all attachements in preview widget

If res_id is not defined, this method used to crash.
Made obvious by commit a26496b6e7,
which added a call to that function in the main controller of web.

opw 1950403

closes odoo/odoo#31923

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>

addons/mail/models/ir_attachment.py

@@ -23,5 +23,5 @@ class IrAttachment(models.Model):
         related_record = self.env[self.res_model].browse(self.res_id)
         # message_main_attachment_id field can be empty, that's why we compare to False;
         # we are just checking that it exists on the model before writing it
-        if hasattr(related_record, 'message_main_attachment_id'):
+        if related_record and hasattr(related_record, 'message_main_attachment_id'):
             related_record.message_main_attachment_id = self