cache key share; support correct traverse of 'of'

cache key share; support correct traverse of 'of'

Fixed caching issue where the
:paramref:`_sql.Select.with_for_update.key_share` element of
:meth:`_sql.Select.with_for_update` was not considered as part of the cache
key, leading to incorrect caching if different variations of this parameter
were used with an otherwise identical statement.

Also repairs a traversal issue where the ``of`` element of
``ForUpdateArg`` when set to ``None`` cannot be compared against a
non-None element because the traversal defines it as a clauselist.
Traversal in this case is adjusted to accommodate for this case so that
we dont need to create a risky-to-backport change to ``ForUpdateArg``
itself.

Fixes: #11544
Change-Id: Ie8a50716df06977af58b0c22a8c10e1b64d972b9