Prevents 400 NVP errors caused by a None display_name

The API forbids a resource name to be None, but the
Model does not. Such errors may  be induced by
programming directly against the plugin interface. With
this fix we avoid raising 400 faults which may be introduced
by involuntary programming errors.

Fixes bug 1221896

Change-Id: Ic1201c5af5691f2bed38753453f73c229858b10f