> What I would expect from a service that kept their word: police knocks at their door, they say "sorry we cannot give you data we don't have, we don't keep records here".

I could see the proper legal (IANAL) answer (for a fully E2E encrypted system) would probably be something like:

    Sure, we will gladly comply as legally obligated to, you will find the whole of the requested data we can provide attached.

    <zip file>

The zip file would contain an index.txt with:

    This is all the content we have access to, which is none at all. This is not a bug, and neither it is a refusal to comply. We do not have access to any other data as it is encrypted when it leaves the user premises.

And maybe a csv file with headers matching the requested data fields but zero rows.

The difference being one doesn't say "no we can't" to law enforcement, instead say "yes, but the result is empty"