Ensuring that AF::Base.find("") raises exception

Prior to this patch, a new object would be instantiated & returned when
ActiveFedora::Base.find("") was called.

Unrelated specs began breaking due to a change in rdf-rdfxml at version
1.0.2
Merge pull request #156 from projecthydra/fix-active-fedora-find-on-blank

Ensuring that AF::Base.find("") raises exception