object-manager: wait until globals appear before emitting "installed"

object managers that are registered a bit early (such as the one in
wireplumber-cli) have no use if they are declared as installed before
any globals appear. After the initial registry startup, there should
be at least 1 global, the core (id=0), so even if this client has
no access to any object, the object manager should be able to
finish its installation successfully