fix(AsyncResult): Fix scalar method error due to missing attribute

fix(AsyncResult): Fix scalar method error due to missing attribute

Fixed bug where :meth:`_asyncio.AsyncResult.scalar`,
:meth:`_asyncio.AsyncResult.scalar_one_or_none`, and
:meth:`_asyncio.AsyncResult.scalar_one` would raise an ``AttributeError``
due to a missing internal attribute.  Pull request courtesy Allen Ho.

Fixes: #12338
Closes: #12339
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/12339
Pull-request-sha: 63ba43365e9624a75e3f206e6b0f4569e3940da6

Change-Id: I44a949e4a942a080338037cd570d4b1dc0d7550d
(cherry picked from commit ca092e73a254a3914fd93ca98340ba7762d4cee9)