Make domain_lookup return consistent type

It is possible to make Horizon reach the "Pure project admin doesn't
have a domain token" state in domain_lookup by having different policy
files in Keystone and in Horizon, at which point it results in an
unhelpful AttributeError later on, because domain_lookup returns None.

I'm proposing to make it return an empty dict instead, this way Horizon
doesn't crash, but we still get the warning in the logs.

Change-Id: I7a42e8defb2fb348cac8225fb1dcbf5e86c9e65c

openstack_dashboard/api/keystone.py

@@ -247,7 +247,7 @@ def domain_lookup(request):
             return dict((d.id, d.name) for d in domains)
         except Exception:
             LOG.warning("Pure project admin doesn't have a domain token")
-            return None
+            return {}
     else:
         domain = get_default_domain(request)
         return {domain.id: domain.name}