Signature issue when fetching actor (GET request)

CF https://social.inex.rocks/@inexcode/103208341765928591:

When I try to follow a library from this server on open.audio, I get a 500 error, probably linked to a signature issue. One of the problem comes from the fact that we're checking the existence of a (request-target) header in federation/signing.py, while it can obviously never exist, but I'm not sure why it wasn't spotted before and doesn't affect existing federation 🤔