pam: add option to not prompt for password if it's set to empty

Add a new option to pam_unix(8), "emptyok", which makes it not prompt
for password, if it's set to an empty one. It is similar to "nullok",
which makes it not prompt for password if the hash itself is empty.

Reviewed By: markj
Sponsored By: NetApp, Inc.
Sponsored By: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D27569