fix: don't barf on errors with blank sqlstate

fix: don't barf on errors with blank sqlstate

Such messages are not entirely valid (sqlstate is documented to be
always present), however we receive them after a SHOW HELP in the
PgBouncer admin database.

The SHOW HELP actually does generate a sqlstate `00000` but the message
is somehow parsed incorrectly by the libpq, which goes on to report an
error:

    message contents do not agree with length in message type "N"

See #303. PgBouncer issue reported upstream in
https://github.com/pgbouncer/pgbouncer/issues/718