Panic when login if user is included in an external group with a nil alias

[uepoch]

Describe the bug
If an external group finish in a state where it has no aliases like this

{
 "request_id": "efe34d71-83d9-5c13-b3a8-f6e46f66a007",
 "lease_id": "",
 "lease_duration": 0,
 "renewable": false,
 "data": {
   "alias": {},
   "creation_time": "2018-08-09T09:33:43.568385967Z",
   "id": "6d02f695-56b0-31bd-d751-484fed91265a",
   "last_update_time": "2018-09-18T11:54:43.297207852Z",
   "member_entity_ids": [
     "b11bfa48-7386-df0b-bd75-1e6fbbd00110"
   ],
   "member_group_ids": null,
   "metadata": {},
   "modify_index": 183,
   "name": "blabla-admin",
   "parent_group_ids": null,
   "policies": null,
   "type": "external"
 },
 "warnings": null
}

(which i'm not sure how it ends in this state, i've read (#5345) maybe it's the same kind of issues)
Then a login from a user that was included in the group members will cause a panic at https://github.com/hashicorp/vault/blob/master/vault/identity_store_util.go#L1871

To Reproduce
(Not sure how to reproduce the nil-ed Alias group as you can't manually include members in external groups)

Expected behavior
No panic

Environment:

• Vault Server Version (retrieve with vault status): 0.11.5, master

Additional context
Add any other context about the problem here.