NPE in case of missing leaf type - fix to prevent li -> Be regression

NETCONF-86 was moved to the netconf project, however there is still an NPE introduced with the new yang rewrite which it was agreed on the MD-SAL call 10/20/2015 should be fixed. I'm creating this with a severity of "Major" as it's very much an issue for development work going on atop Be ... it's not "normal" and the workaround is not acceptable.

Work is underway :

https://git.opendaylight.org/gerrit/28511 is