sysctl: emit a newline after NULL node descriptions

Previously when printing the sysctl description (via the -d flag) we
omitted the newline if the node provided no description (i.e., NULL).
This could be observed via e.g. sysctl -d dev.

PR: 44034
Reviewed by: zlei
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42112