Deprecate empty or_() and and_()

Creating an :func:`.and_` or :func:`.or_` construct with no arguments or
empty ``*args`` will now emit a deprecation warning, as the SQL produced is
a no-op (i.e. it renders as a blank string). This behavior is considered to
be non-intuitive, so for empty or possibly empty :func:`.and_` or
:func:`.or_` constructs, an appropriate default boolean should be included,
such as ``and_(True, *args)`` or ``or_(False, *args)``.   As has been the
case for many major versions of SQLAlchemy, these particular boolean
values will not render if the ``*args`` portion is non-empty.

As there are some internal cases where an empty and_() construct is used
in order to build an optional WHERE expression, a private
utility function is added to suit this use case.

Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Fixes: #5054
Closes: #5062
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/5062
Pull-request-sha: 5ca2f27281977d74e390148c0fb8deaa0e0e4ad9

Change-Id: I599b9c8befa64d9a59a35ad7dd84ff400e3aa647

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