Fix with_expression() cache leak; don't adapt singletons

Fix with_expression() cache leak; don't adapt singletons

Fixed a cache leak involving the :func:`_orm.with_expression` loader
option, where the given SQL expression would not be correctly considered as
part of the cache key.

Additionally, fixed regression involving the corresponding
:func:`_orm.query_expression` feature. While the bug technically exists in
1.3 as well, it was not exposed until 1.4. The "default expr" value of
``null()`` would be rendered when not needed, and additionally was also not
adapted correctly when the ORM rewrites statements such as when using
joined eager loading. The fix ensures "singleton" expressions like ``NULL``
and ``true`` aren't "adapted" to refer to columns in ORM statements, and
additionally ensures that a :func:`_orm.query_expression` with no default
expression doesn't render in the statement if a
:func:`_orm.with_expression` isn't used.

Fixes: #6259
Change-Id: I5a70bc12dadad125bbc4324b64048c8d4a18916c