test-qobject-input-visitor: Use strict visitor

test-qobject-input-visitor: Use strict visitor

The qobject input visitor comes in a strict and a non-strict variant.
This test is the non-strict variant's last user.  Turns out it relies
on non-strict only in test_visitor_in_null(), and just out of
laziness.  We don't actually test the non-strict behavior.

Clean up test_visitor_in_null(), and switch to the strict variant.
The next commit will drop the non-strict variant.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1488544368-30622-19-git-send-email-armbru@redhat.com>