fix(smoke-test): gate tx-generator block on compose having the service
After PR #111 dropped tx-generator from cardano_node_master/, the 'Compose smoke test' job that runs scripts/smoke-test.sh against master fails because the script unconditionally probes the tx-generator control socket — which is only present on the new cardano_node_tx_generator/ testnet now. Skip the tx-generator-specific block when the compose under test doesn't declare a 'tx-generator:' service. Same script keeps working for both master (no tx-generator) and the new feature testnet (still drives the daemon end-to-end).