procstat_getfiles_sysctl: do not require non-null ki_fd

ki_fd is legitimately NULL when 32bit process requests process data
from 64bit host kernel. The field is not used by the code for sysctl
case; procstat_getfiles_kvm() checks ki_fd.

PR: 260174
Reported by: Damjan Jovanovic <damjan.jov@gmail.com>
Sponsored by: The FreeBSD Foundation
MFC after: 1 week