Add an option to allow an inventory that does not define any host

The use case for this is when hosts are first dynamically detected/provisionned on a local play and added to the inventory with add_host.

Currently, we can use the "comma hack", but:
1) it's a hack and may disappear
2) it does not allow the use of a group_vars directory at the same level as the inventory file (the comma makes is_file return false, so no basedir is set, and then no basedir is searched)