WFLY-3881 User shouldn't be able to use empty JDNI name ('java:/' or 'java:jboss/')

https://issues.jboss.org/browse/WFLY-3881
https://bugzilla.redhat.com/show_bug.cgi?id=1019260

Tested.
Isn't any JNDI name which ends with a slash "/" invalid here?  I.e. you need a concrete name, not a context.