cql: Reject empty options while altering a keyspace

Reject ALTER KEYSPACE request for NetworkTopologyStrategy when
replication options are missed.

Also reject CREATE KEYSPACE with no replication factor options.
Cassandra has a default_keyspace_rf configuration that may allow such
CREATE KEYSPACE commands, but Scylla doesn't have this option (refs #16028).

fixes #10036

Closes scylladb/scylladb#16221

locator/network_topology_strategy.cc

@@ -256,6 +256,9 @@ network_topology_strategy::calculate_natural_endpoints(
 }
 
 void network_topology_strategy::validate_options(const gms::feature_service& fs) const {
+    if(_config_options.empty()) {
+        throw exceptions::configuration_exception("Configuration for at least one datacenter must be present");
+    }
     validate_tablet_options(fs, _config_options);
     auto tablet_opts = recognized_tablet_options();
     for (auto& c : _config_options) {