Convertor warns about missing license when converting SPDXSimpleTag.tag to rdf

[nicoweidner]

I am getting the following warning:

$ convertor -f tag data/SPDXSimpleTag.tag -o output.rdf
/home/nico/IdeaProjects/tools-python/venv/lib/python3.8/site-packages/spdx/writers/rdf.py:148: UserWarning: Missing extracted license: LicenseRef-2.0
  warnings.warn(

Not entirely sure what this warning is about, so I will leave it in this issue for now. It is not shown for other output formats. The created rdf file does contain a reference to the license, though without text:

        <ns1:licenseConcluded>
          <ns1:ConjunctiveLicenseSet rdf:nodeID="N7b9b488d98c44779a852cbe0985ac244">
            <ns1:member rdf:resource="http://spdx.org/licenses/Apache-2.0"/>
            <ns1:member>
              <ns1:ExtractedLicensingInfo rdf:nodeID="N4efe760119e54e0c9ccaa773e3d1fd07">
                <ns1:licenseId>LicenseRef-2.0</ns1:licenseId>
                <ns1:extractedText>None</ns1:extractedText>
                <ns1:licenseName>LicenseRef-2.0</ns1:licenseName>
              </ns1:ExtractedLicensingInfo>
            </ns1:member>
          </ns1:ConjunctiveLicenseSet>
        </ns1:licenseConcluded>