Modify Account class to handle NULL references
This patch modifies the Account class to defer to using the associated
domain's tid/ssid/qid fields if the per-instance fields are None/NULL.
# All parameters are described in pgsql_table(5) / PGSQL PARAMETERS
#
# The hosts that Postfix will try to connect to and query from.
hosts = localhost
# The user name and password to log into the pgsql server.
user = postfix
password = some_password
# The database name on the servers.
dbname = mailsys
# The SQL query template used to search the database
query = SELECT destination FROM postfix_virtual_alias_map('%u', '%d')