pam_unix: Store empty password as empty hash

[zirias]

Summary

pw(8) normally refuses setting an empty password, but offers the
-w none option to do that, representing the empty password by an empty
stored hash. pam_unix allows setting an empty password with the "nullok"
option, but so far stored a hash of the empty string for it. Change this
to store an empty hash as well for consistency.

[zirias]

rebase on updated parent revision