cache: real_dirty_memory_accounter: Allow construction without memtable
cache: real_dirty_memory_accounter: Allow construction without memtable

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