bsdinstall: don't fail if fstab is empty

PR: 210865
Submitted by: Tim Lukasiewicz <eclipzetech08@gmail.com>, h-fujishima@sakura.ad.jp (original version)
Reported by: h-fujishima@sakura.ad.jp
Reviewed by: sef
MFC after: 2 weeks
Sponsored by: Klara Inc.
Event: July 2020 Bugathon