If I create an account, and don't give it a password, what's the password to SSH it?
Having no password on the OS level effectively means that ssh is disabled for that user. The protocol insists on a non-zero password and thus the hashes of whatever password is supplied fails to match the absence of a hash.
You must supply ssh keys or otherwise arrange some authentication to log in should your account have no password.
Interesting question. I don't know the answer, but if OS X behaves like other UNIXes (and I suspect it does), it is possible to for a userid to exist without a password. In that case, the login is disabled and incoming ssh connections will prompt for a password but nothing will satisfy the requirement to authenticate.