sorry but I am not aware that there has to be a democratic candidate for office.
A quick description of the electoral college process. The DEM party is on the ballot in all 50 states. When you and I vote, we vote for electors that have already been chosen by the parties. In December, the electors cast their ballots for the candidate that the party tells them to cast for.
So, you are correct. There does NOT have to be a candidate on the ballot, in November. But that does not mean the "no candidate" party won't put their (to be determined in time) person in the office.