storage_proxy: do not crash on LOCAL_QUORUM access to a DC with zero replication
storage_proxy: do not crash on LOCAL_QUORUM access to a DC with zero replication

If a table that is not replicated to a certain DC (rf=0) is accessed
with LOCAL_QUORUM on that DC the current code will crash since the
'targets' array will be empty and read executor does not handle it.
Fix it by replying with empty result.

Fixes #8354

Message-Id: <76795f38479cef3be1bab6f7448356ba02c5a9e3@scylladb.com>

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