Handle the odd case no package is needed in roles/dbserver-pgsql/server
If any package has already been installed at build step on FreeBSD,
and postgresq-contrib isn't needed, we don't have anything to install.
This is a follow-up for e79d77e83a85.
Test Plan: Tested on db-A-001
Reviewed By: dereckson
Maniphest Tasks: T1888
Differential Revision: https://devcentral.nasqueron.org/D3132