Publish messages to None topic with AIOKafkaProducer

Hello,
If we try publish message into `None` topic there are some error messages:
```
Unable to request metadata from node with id 0: ConnectionError: Connection at localhost:9092 broken
Unable to update metadata from [0]
...
```

Code snippet to reproduce:
```python
import aiokafka
p = aiokafka.AIOKafkaProducer(bootstrap_servers=["localhost:9092"],
loop=loop)
loop.run_until_complete(p.start())
loop.run_until_complete(p.send_and_wait(None, b"sobaka"))
loop.run_until_complete(p.stop())
```
Packages versions:
```
aiokafka==0.4.0
kafka-python==1.3.5
```