RandomString RefId return name with no resource_id

Prior to https://review.openstack.org/134557 we'd return the name
when resource_id was not set (e.g during validation), whereas now
we'll return None.  So reinstate the behavior of the base-class
FnGetRefId in the overridden function.

Change-Id: I233476b8411612350c884c4480a8e2f5cc8c2ab5
Partial-Bug: #1392359
Merge "RandomString RefId return name with no resource_id"