etcdserver: don't attempt to grant nil permission to a role

Prevent etcd from crashing when given a bad grant payload, e.g.:

$ curl -d '{"name": "foo"}' http://localhost:2379/v3/auth/role/add
{"header":{"cluster_id":"14841639068965178418", ...
$ curl -d '{"name": "foo"}' http://localhost:2379/v3/auth/role/grant
curl: (52) Empty reply from server

server/etcdserver/api/v3rpc/util.go

@@ -77,6 +77,7 @@ var toGRPCErrorMap = map[error]error{
 	auth.ErrRoleNotFound:         rpctypes.ErrGRPCRoleNotFound,
 	auth.ErrRoleEmpty:            rpctypes.ErrGRPCRoleEmpty,
 	auth.ErrAuthFailed:           rpctypes.ErrGRPCAuthFailed,
+	auth.ErrPermissionNotGiven:   rpctypes.ErrGRPCPermissionNotGiven,
 	auth.ErrPermissionDenied:     rpctypes.ErrGRPCPermissionDenied,
 	auth.ErrRoleNotGranted:       rpctypes.ErrGRPCRoleNotGranted,
 	auth.ErrPermissionNotGranted: rpctypes.ErrGRPCPermissionNotGranted,