partition_version: Make operator bool() const-qualified
partition_version: Make operator bool() const-qualified

[tgrabiec: Extracted from a different patch]

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