Make summarize_protover_flags() handle NULL and empty string the same
summarize_protover_flags(NULL, NULL) doesn't set protocols_known, but summarize_protover_flags("", "") does.
While this situation probably won't happen in practice, it could be a source of subtle bugs.
And we have a general guideline that functions should treat NULL and "" in similar ways. (Or the difference should be clearly documented.)
So we should ignore "" protovers, the same way we ignore NULL protovers. (Relays with empty protovers won't end up in the consensus, and clients can't use them for anything. So this change should have no real impact.)