base: support zero argument exports.

* specs/base_spec.yaml (export): Remove specification for zero
argument export error.
* lib/std/base.lua (export): Remove zero argument error.
* specs/base_spec.yaml (export): Specify argument checking of
zero argument exports.
* lib/std/base.lua (export): Support zero arguments.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>