Empty elements throwing TypeErrors

[mrmiguez]

For example

        <subject>
          <name authority="lcnaf" authorityURI="http://id.loc.gov/authorities/names" type="personal" valueURI="http://id.loc.gov/authorities/names/n85385724">
            <namePart type="date"/>
            <namePart type="given">Benjamin V.</namePart>
            <namePart type="family">Cohen</namePart>
          </name>
        </subject>

returns KeyError - sourceResource.subject: None, Can't convert 'NoneType' object to str implicitly