legacy_schema_migrator: Don't snapshot empty legacy tables
legacy_schema_migrator: Don't snapshot empty legacy tables

Otherwise we will create a new (empty) snapshot each time we boot.
Message-Id: <46b2e2c8e9add14c89e0e0d8e60f2c823ac14a59@scylladb.com>

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