bpo-44469: Fix tests for "async with" with bad object (GH-26817)

Test for execution of the body was null. It would pass
even if the code which should be skipped was executed.
(cherry picked from commit 5d2b3a0d688cf8a33db3d266c9e7049c13766a4c)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>