AvroProducer doesn't serialize the key if it's an empty string

[alfiya400]

Description

Based on the code for AvroProducer
AvroProducer serializes the key only if bool(key) == True, which allows to pass an empty string as a key without the serialization.

This breaks the read using AvroConsumer, because it tries to deserialize the key if message.key() is not None
And the serializer raises the following error at the length check

Traceback (most recent call last):
  File "/Users/alfiya/miniconda3/envs/py3/lib/python3.6/site-packages/IPython/core/interactiveshell.py", line 2862, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-20-7566bd90b3a5>", line 1, in <module>
    m=consumer.poll()
  File "/Users/alfiya/miniconda3/envs/py3/lib/python3.6/site-packages/confluent_kafka/avro/__init__.py", line 118, in poll
    decoded_key = self._serializer.decode_message(message.key())
  File "/Users/alfiya/miniconda3/envs/py3/lib/python3.6/site-packages/confluent_kafka/avro/serializer/message_serializer.py", line 209, in decode_message
    raise SerializerError("message is too small to decode")

How to reproduce

import avro.schema
import confluent_kafka.avro

producer = confluent_kafka.avro.AvroProducer(
    {
        'api.version.request': True,
        'compression.codec': 'gzip',
        'default.topic.config': {'request.required.acks': 1},
        'bootstrap.servers': 'localhost:9093',
        'schema.registry.url': 'http://localhost:8081',
    }, 
    default_key_schema=avro.schema.Parse('{"type": "string"}'), 
    default_value_schema=avro.schema.Parse('{"type": "string"}')
)
producer.produce(topic='bug', value='not empty', key='')

consumer = confluent_kafka.avro.AvroConsumer({
    'group.id': 'mygroup', 
    'api.version.request': True,
    'default.topic.config': {'auto.offset.reset': 'smallest'},
    'bootstrap.servers': 'localhost:9093',
    'schema.registry.url': 'http://localhost:8081'
    
})
consumer.subscribe(['bug'])
consumer.poll()

At consumer.poll() the error above should be raised:

Checklist

Please provide the following information:

• confluent-kafka-python and librdkafka version (confluent_kafka.version() and confluent_kafka.libversion()): confluent_kafka.version(): ('0.11.0', 720896), confluent_kafka.libversion(): ('0.11.3', 721919)
• Apache Kafka broker version: 0.10.2.1
• Client configuration: {...} provided in 'How to reproduce'
• Operating system: Mac OS X 10.13.3
• Provide client logs (with 'debug': '..' as necessary)
• Provide broker log excerpts
• Critical issue

[rnpridgeon]

For completeness:

The AvroProducer will actually fail to serialize both the key and the value if they are represented by an empty string. This is actually true for 0 values on all the numeric types and empty sequences as well per the python documentation.

The difference being 0 numeric types will actually fail to be written at all as it will fail the type checking done by the producer. Had it been serialized it would have been converted to an array of bytes.

Instead the AvroProducer should us the object identity operators is not to evaluate the presence of a key and/or value.

https://docs.python.org/2/library/stdtypes.html#truth-value-testing

[rnpridgeon]

Fixed on master (#374)