sysctl: emit a newline after NULL node descriptions

[emaste]

Summary

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

(PR is a report of sysctls with empty descriptions, which is how I found this issue.)

[emaste]

Before:

$ sysctl -d dev | head -n 2
dev: dev.hconf: 
dev.hconf.0: 

After:

$ sbin/sysctl/obj/sysctl -d dev | head -n
2
dev: 
dev.hconf: 

Possible test addition:

atf_test_case sysctl_dflag_NULL
sysctl_dflag_NULL_head()
{
       atf_set "descr" "Verify newline after NULL description"
}
sysctl_dflag_NULL_head()
{
       atf_check -s exit:1 sysctl -d dev | head -n 1 | grep ":"
}

[zlei]

Looks good to me.