Handle the odd case no package is needed in roles/dbserver-pgsql/server

Summary:
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.

Ref T1888

This is a follow-up for e79d77e83a85.

Test Plan: Tested on db-A-001

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1888

Differential Revision: https://devcentral.nasqueron.org/D3132