pfctl: fix no nat / no rdr rules

In aeddee83341 the nat rule parsing was significantly reworked, unintentionally
breaking no nat / no rdr rules. The option to not have a -> ... redirspec was
omitted.

Add trivial test cases to catch such issues in the future.

Reported by: Thomas Pasqualini <thomas.pasqualini@orange.com>
Sponsored by: Orange Business Services
Differential Revision: https://reviews.freebsd.org/D49665