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