async with AIOKafkaProducer() fails with AttributeError

**Describe the bug**
```python
async with AIOKafkaProducer(...) as producer:
producer.send(...)
```

fails with `AttributeError: 'NoneType' object has no attribute 'send'`. `producer` is indeed `None`.

**Expected behaviour**

`producer` is an `AIOKafkaProducer` object.

**Environment (please complete the following information):**
- aiokafka version (`python -c "import aiokafka; print(aiokafka.__version__)"`): 0.7.0
- kafka-python version (`python -c "import kafka; print(kafka.__version__)"`): 2.0.2
- Kafka Broker version (`kafka-topics.sh --version`): 2.6.0
- Other information (Confluent Cloud version, etc.): none relevant

**Reproducible example**
```python
async with AIOKafkaProducer(...) as producer:
producer.send(...)
```