This is meant to handle a quirk in which key descriptors obtained
through walletrpc.DeriveKey don't result in the derived key being
persisted to the wallet's database, unlike with DeriveNextKey. Due to
this and some fallback logic in the wallet with regards to empty key
locators, if a request only specified the compressed public key, the
signature returned would be over a different key, namely the one derived
from (family=0, index=0).
