The IP, TCP, and UDP provider report IP addresses as strings.
In some cases, the required information is not available and the
UDP provider reported an empty string in this case and the IP
and TCP provider reported a NULL pointer.

This patch changes the value provided in this case to the string
"<unknown>". This make the behaviour consistent and in-line with
the behaviour of Solaris.

Reviewed by: markj@, dteske@, gnn@
Differential Revision: https://reviews.freebsd.org/D15855