mutation_reader: Make empty mutation source advertize no partitions
mutation_reader: Make empty mutation source advertize no partitions

So that perf_row_cache_update will always populate cache.

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