Not voting for the republican nominee, whoever that may be, is the same is basically giving half a vote for Hilary. It is still your choice, and if you truly have no preference between the two then stay home. But, if you do, you are essentially voting for Hilary- which is you right to do. But to claim that staying home is in no way voting for Hilary is not truthful. If you are not adding to the total number of votes for one side, then you are essentially adding to the total for the other.
Uh, you could simply replace 'Hillary' with 'Trump' and come to the same conclusion by that logic.