It's down ticket only for me, even if there was an actual Conservative running this year it would make no difference if I voted because California is going to the demonrat.
I would still recommend for President, finding the candidate available on your ballot that most closely matches your values, even if it is third party or write-in versus leaving that blank.
This article specifically uses California as an example and they show the mathematical weight of your one vote in California for that candidate is far more powerful than a vote for a major party candidate.