fix(kafkajs): handle kafka tombstone messages

[david-luna]

The kafkajs instrumentation was calling toString method for all of the messages received. This approach wasn't taking in consideration tombstone messages which is defines as

A message with a key and a null payload will be treated as a delete from the log. Such a record is sometimes referred to as a tombstone.

Ref: https://kafka.apache.org/documentation/#design_compactionbasics

Fixes: #3980

Checklist

• Implement code
• Add tests
• Update TypeScript typings
• Update documentation
• Add CHANGELOG.asciidoc entry
• Commit message follows commit guidelines

[trentm]

/test tav kafkajs