[FIX] mail: upload_attachment without model

The method /web/binary/upload_attachment may be used (eg. it is the case
in enterprise documents module) without res_model (eg. if folder_id is
specified).

Thus we should make it work (that was not possible since a26496b6).

opw-1964091
opw-1965832
opw-1967181
opw-1968063
closes #32692

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

addons/mail/models/ir_attachment.py

@@ -20,6 +20,8 @@ class IrAttachment(models.Model):
         attached to.
         """
         self.ensure_one()
+        if not self.res_model:
+            return
         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