suppress log error when $user unset
suppress log error when $user unset

test for and return earlier when a null sender is encountered.
Prevents using an undefined variable.

priyadi/qpsmtpd,smtpd/qpsmtpd,msimerson/qpsmtpd,priyadi/qpsmtpd,msimerson/qpsmtpd,sieutruc/qpsmtpd,sieutruc/qpsmtpd,salvis/qpsmtpd,salvis/qpsmtpd,smtpd/qpsmtpd,sieutruc/qpsmtpd,priyadi/qpsmtpd