avoid throwing exceptions when namespace function called with nil

[mprokopov]

When the null value supplied to the namespace function execution doesn't hit the second parameter to or, namely string "unnamespaced" because it throws Exception ClassCastException for null.

Could be fixed by wrapping in try-catch block, but probably using and is a bit simpler.

[delyada]

Schema Cartographer wasn't working for us either on our still small schema, failing with a NullPointerException (I suspect the op typoed ClassCastException while meaning the NPE.) After a debugging session I reached the same cause and @mprokopov's solution did it for us too. Thanks!