First, your AND statment:
AND pr_projet.id_proj = 0_contact.id_proj
use 0_contact.id_proj, but that field is not listed in the table structure, so I'm assuming it should be there.
Second, here is my best guess at to what will help.
SELECT pr_projet.lib_proj, 0_contact.nom_contact, 0_client.nom_client
from pr_projet left outer join 0_contact on pr_projet.id_proj = 0_contact.id_proj
left outer 0_client on pr_projet.id_client = 0_client.id_client
Also, since you do not have an aggregate field (count, sum, max, min), you do not need a group by clause.
Hope this helps.
Tim