Found another post discussing this.. doesn't seem to have a solid answer there either.
http://www.sqlservercentral.com/Forums/Topic982141-391-1.aspx
One recommendation I saw there was to remove the endpoint from the witness, and then re-create it. Drawback might be that it will clear out all existing witness information, not just the NULLS.
Basically, looks like an orphaned witness config, and dropping the endpoint might clear up the whole dmv.
I haven't tested any of that, all speculation.
Regards,
Steve