What if they gave an election and nobody came?
I have read of people whose conscience won't let them vote for Trump. They say they'll vote in the down-ticket races, just not the presidency.
On the other hand, if you don't vote for Trump, it's alleged to be a vote for Hillary. And if you don't vote for Hillary, it's like voting for Trump. If you vote for a third person altogether, it's wasting your vote, unless you believe in making statements or in miracles. So, conscience voter, I understand your quandary. I don't blame you for not wanting to vote for a president at all.
Someone will still be elected. The Oval Office won't be left empty when Obama departs next January. Someone is going to be our chief executive and commander-in-chief – probably someone utterly unfit for and unsuited to steering our nation through current and future crises.
I cannot recommend how you should cast your vote. Not voting, however, does not exempt you from responsibility for the outcome. It would be a sin of omission, in that you could have done a good thing (i.e., prevent a worse thing) and chose not to.
Will there ever be an election where either or both candidates are ideal? No. We all have different opinions as to which is the most important issue, and what is the optimal course of action to deal with it. We make compromises in every election. We rank the issues in the order of our concern and see which candidate is closest to what we wish for. It's not always a formal process, and some people settle for finding one issue where a candidate has a position they can support.
With Clinton and Trump, finding one or more such issues is perhaps more difficult than it has ever been. We didn't expect Trump to get the Republican nomination. We hoped, and still do, for an indictment of Clinton instead of an inauguration, but it is what it is.
If your conscience can accept the consequences of not voting, perhaps failing to vote for the least terrible candidate is the right choice for you. My conscience requires that I suck it up and vote for Trump. I despise Trump, but voting third party seems more dangerous. If a miracle fails to happen, Hillary will take office and claim a mandate from the people.
Regarding the title question, the answer is that a Democrat would vote for himself and declare a landslide victory. After all, even if no one shows up, Democrats get the majority of the dead vote.