Skip second WriteToc() call for custom-format dumps without data.

Skip second WriteToc() call for custom-format dumps without data.

Presently, "pg_dump --format=custom" calls WriteToc() twice.  The
second call updates the data offset information, which allegedly
makes parallel pg_restore significantly faster.  However, if we're
not dumping any data, there are no data offsets to update, so we
can skip this step.

Reviewed-by: Jeff Davis <pgsql@j-davis.com>
Discussion: https://postgr.es/m/Z9c1rbzZegYQTOQE%40nathan