Nobody, I started watching in October 1998 and the title had been vacant for a month. I started watching after Judgement Day 1998 and of course The Rock was crowned champion at Survivor Series.
That's something I would love for them to do at some point, the World Champion faces the WWE Champion in a unification match which ends in a no contest. Rather than continue the match or re-start it the title ends up vacated. The next PPV is a one night tournament to decide the Undisputed WWE Champion with a new face winning the title who has never been champion before.