its not up to them to prove there is encrypted data, its up to you to prove there isn't.

or convince the court that you have genuinely forgotten the passphrase, all they have to do is find a file they cannot read, claim its encrypted and force you to decode it for them.

note I'm assuming that if you do decode the file, and its not what they expected its now up to you to prove there isn't anything else in there as well.

wonderful law. guilty until proven innocent, and proven to a standard thats a moving target.