Cast empty PostgreSQL ARRAY from the type specified to array()

Cast empty PostgreSQL ARRAY from the type specified to array()

When building a PostgreSQL ``ARRAY`` literal using
:class:`_postgresql.array` with an empty ``clauses`` argument, the
:paramref:`_postgresql.array.type_` parameter is now significant in that it
will be used to render the resulting ``ARRAY[]`` SQL expression with a
cast, such as ``ARRAY[]::INTEGER``. Pull request courtesy Denis Laxalde.

Fixes: #12432
Closes: #12435
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/12435
Pull-request-sha: 9633d3c15d42026f8f45f5a4d201a5d72e57b8d4

Change-Id: I29ed7bd0562b82351d22de0658fb46c31cfe44f6