Spdx2to3Converter: return null for "NONE" or "NOASSERTION" Agents

[tiegz]

The javadocs for Spdx2to3Converter.stringToAgent() say:

• @return Agent based on parsing the spdx2personOrgString - if NONE or NOASSERTION is the string value, the null is returned

But null is not being returned for NONE or NOASSERTION strings, so the method is creating a new "Agent" node for each time these agent values are needed, e.g. for a "suppliedBy" value:

...
  }, {
    "spdxId" : "https://org.spdx.spdxdata/4320c18a-dbd2-49ba-9ca7-c0e5230700a6/SPDXRef-gnrtd75",
    "type" : "Agent",
    "name" : "NOASSERTION",
    "creationInfo" : "_:creationInfo_0"
  }, {
    "spdxId" : "https://org.spdx.spdxdata/4320c18a-dbd2-49ba-9ca7-c0e5230700a6/SPDXRef-gnrtd83",
    "type" : "Agent",
    "name" : "NOASSERTION",
    "creationInfo" : "_:creationInfo_0"
  }, {
    "spdxId" : "https://org.spdx.spdxdata/4320c18a-dbd2-49ba-9ca7-c0e5230700a6/SPDXRef-gnrtd91",
    "type" : "Agent",
    "name" : "NOASSERTION",
    "creationInfo" : "_:creationInfo_0"
  }, {
...

This fix returns null instead so the convert won't create the "Agent" nodes.

[goneall]

Thanks @tiegz for the PR.

I looked back through the history and it looks like the check for NONE and NOASSERTION was never implemented, so I don't expect there to be any issues with implementing the original intent based on the comments.