[Exim] mysql expansion

Some people have already told me that this problem was discussed on this
mailing list long before but I couldn't find that message. I have this
router:

mysql_user:
   driver = accept
   condition = \
    ${if eq{} \
      { ${lookup mysql {SELECT maildir FROM users \
          WHERE id='${local_part}@${domain}'}} \
      } \
      {no}{yes} \
    }
#  retry_use_local_part
   transport=mysql_delivery

the problem is that exim thinks that condition is true even if mysql
returns empty string (that actually means user doesn't exist). How
should I change the condition in order exim treats mysql answer properly?