Turns out that we think this is related to legacy/trac#27774. A client selects a v3 RP by only looking at the protover but it is possible that at that time we simply don't have the descriptor yet thus missing the `onion_pk` which led to a failure when sending the INTRODUCE1 cell. Like `node_supports_ed25519_link_authentication()` does, we should simply check for the `onion_pk` as an extra step to the protover. Flagging this for backport.