tombstone messages on a topic throws off agent commit offsets

[patkivikram]

Checklist

• [ x] I have included information about relevant versions
• [x ] I have verified that the issue persists when using the master branch of Faust.

Steps to reproduce

Create a topic and send multiple messages on the topic using any cli tool as topic.send(key="k1", value=null)

Bring up the faust agent to process this topic and notice that none of these messages are acked . If non null messages are sent on this topic they are never acked

Expected behavior

The commit offset should progress and the null/tombstone message must be processed by the agent

Actual behavior

The message is swallowed by the faust library, but never processed and acked by the agent

Full traceback

Paste the full traceback (if there is any)

Versions

• Python version -3.7
• Faust version 1.7.4, 1.8
• Operating system centos
• Kafka version 2.0
• RocksDB version (if applicable) 6.1