bucket created on 7.0 node breaks authentication

Mixed cluster node1 (6.6), node2 (7.0)
bucket "test" created on node1
bucket "test1" created on node2
(n_0@192.168.0.9)3> ns_config_auth:authenticate("test", "asdasd").
false
(n_0@192.168.0.9)4> ns_config_auth:authenticate("test1", "asdasd").

• • exception error: no function clause matching misc:compare_secure("asdasd",undefined) (src/misc.erl, line 2781)
    in function ns_config_auth:authenticate/2 (src/ns_config_auth.erl, line 111)

curl -v http://test1:asdasd@localhost:9000/pools/default => 500
curl -v http://test:asdasd@localhost:9000/pools/default => 401

broken by this change:
http://review.couchbase.org/c/ns_server/+/149105/3/src/menelaus_web_buckets.erl#b453

to fix it we need to make sure that buckets created on 7.0 node in mixed cluster still have