Fix an issue where XMPP accounts weren't reading passwords from the credential manager

XMPP has passwords set to optional because of the SASL implementation. However,
the fallback for that SASL implementation wasn't checking the credential manager
for a password which lead to a prompt for the password. This patch corrects that
behavior.

Testing Done:
Connected an XMPP account with the `require password` account option set to `FALSE` with a password stored in the active credential provider and verified that it successfully logged into without prompting for a password.

Reviewed at https://reviews.imfreedom.org/r/1969/