message: [docs] handle empty mini_ds from kafka during online training
diff: " mini_ds = mini_ds.shuffle(buffer_size=32)\n",
  " mini_ds = mini_ds.map(decode_kafka_online_item)\n",
  " mini_ds = mini_ds.batch(32)\n",
  - " model.fit(mini_ds, epochs=3)\n"
  + " if len(mini_ds) > 0:\n",
  + " model.fit(mini_ds, epochs=3)"
  ]
  },
  {
  