"I only ever cross borders with a blank phone because I don’t want you invading my privacy" is a perfectly valid answer. You can also add that it is your employer’s policy and/or your government official recommendation.
> a perfectly valid answer
Makes no difference at all in the real world. You don't have to give valid answers, you need to get the guy across from you to not find you suspicious. That phrase is going to put a red flag on you, valid or not.
It's one that will get you denied entry, or detained indefinitely.
You can also point out that other countries want to search phones too.
"I have to do this because of country X, you know that they're like, amirite?"