ctld: Ports without a portal group are not dummy ports

The default implementation of is_dummy should return false. Only
portal group ports should possibly return true.

PR: 293076
Reported by: Ken J. Thomson <thomsonk@yandex.com>
Fixes: 6acc7afa34aa ("ctld: Convert struct port to a hierarchy of C++ classes")
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D56524