I don’t see how it is a logical fallacy. A fallacy is when the conclusion is not supported by the premises. “Nothing to hide” is only one of those two required elements, the premise. The conclusion is undefined and might or might not be supported by the premise.

“Nothing to hide” is often a fallacy when arguing, say, government surveillance. “If you have nothing to hide you’ll accept metal detectors at the airport” is a fallacy. I accept metal detectors, but it’s because I value everyone’s safety over my minor invasion of privacy, I don’t think the premise of “nothing to hide” leads to the conclusion.

But I can’t say for sure every “nothing to hide” argument would be a fallacy without the rest of the argument.