The issue with the 'nothing to hide' argument is that it puts the burden of proof and determination of whether something is 'hide-worthy' on the target of the inquiry.
If you subscribe to the 'presumed innocent' premise of the law (https://en.wikipedia.org/wiki/Presumption_of_innocence) then the burden of proof is on the inquisitor.
Either you believe in presumed innocence or you don't. Pick one.