[FIX] mail: no insert message when delete attachment from composer

Before this commit, when deleting an attachment from composer,
an undefined message was inserted in the store.

closes odoo/odoo#125313

X-original-commit: ec814f14222509caa27d627a193e6eaebe9bbf60
Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/mail/models/ir_attachment.py

@@ -42,7 +42,7 @@ class IrAttachment(models.Model):
         if message:
             message.write({})  # to make sure write_date on the message is updated
         self.env['bus.bus']._sendmany((attachment._bus_notification_target(), 'ir.attachment/delete', {
-            'id': attachment.id, 'message': {'id': message.id, 'write_date': message.write_date} if message else {}
+            'id': attachment.id, 'message': {'id': message.id, 'write_date': message.write_date} if message else None
         }) for attachment in self)
         self.unlink()