Missing password causes SCRAM auth to give confusing and unhelpful error message

[sehrope]

If the server requests SCRAM authentication but the user has not supplied one, the current code throws an internal error as we try to cast the null password value to a not-null one for use in the SCRAM handshake.

We need to add a check similar to the MD5 one to ensure that the user has actually supplied a password.

Originally reported on the postgresql-jdbc mailing list:

https://www.postgresql.org/message-id/flat/CAHsP%3D4_aCd13LMCgZnaeN_%2B3BxkM9etU5HZ3%3Dt5UV36h73cHkg%40mail.gmail.com#c83ef9c8d5d9f2d961a2a84746f2cc53