storage_proxy: do not touch all_replicas.front() if it's empty.
storage_proxy: do not touch all_replicas.front() if it's empty.

The list of all endpoints for a query can be empty if we have
replication_factor 0 or there are no live endpoints for this token.
Do not access all_replicas.front() in this case.

Fixes #5935.
Message-Id: <3612e2f64b430f2366b3f7d0f9db3170f89d8fff@scylladb.com>

scylladb/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla