Fix control batch bug

[gkousouris]

Problem

_unpack_message_set was modified in #72 to skip over transactional control batches.

Because of this, unpacked = _unpack_message_set may return an empty list.
However, later in the code we reference the last element of that list, unpacked[-1], to find how many messages have been consumed. This causes a list index out of range exception

Solution

To solve this, we do not want to reference unpacked[-1], but skip the message.

Testing

I have manually edited the file to this from within the venv and this error was fixed:

{...}/venv/lib/python3.10/site-packages/kafka/consumer/fetcher.py", line 830, in _parse_fetched_data
    last_offset = unpacked[-1].offset
IndexError: list index out of range
exception 'IndexError('list index out of range')' encountered running paastorm.{..}`

[JayH5]

LGTM. Do you want to bump the version in this PR?

[gkousouris]

LGTM. Do you want to bump the version in this PR?

ah, might as well bump it now