tls: checking if dhparam is empty, using "undef" instead of "" for clarity.
tls: checking if dhparam is empty, using "undef" instead of "" for clarity.

Signed-off-by: Tom Li <1f43714c5a8c7a5a6c99f9492624e2b771b1f39c@gmail.com>

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