193 import arrayref bug

The problems described in #193 occurred because of the presence of this element in the problematic import file, while defining a new digital object:

                    <applications>
	                        <checksum>Advanced Checksum Verifier</checksum>
	                       
	                        <virusCheck>OfficeScan Client</virusCheck>
	                        <other>
	                        </other>
                    </applications>

The importer didn't gracefully handle the existence of that empty-but-for-whitespace `<other>` element. This change modifies the importer such that the presence of an empty element like this will be interpreted as "clear away all of the many-to-many relationships described by this element, and don't replace them with anything."