bhyve: ahci: Fix regression with no ports

An AHCI controller may be specified with no connected ports. Avoid
dumping core in this case for compatibility with existing VM configs.

Reviewed by: khng, jhb
Fixes: 621b5090487de Refactor configuration management in bhyve.
MFC after: 1 week
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D33969