Create empty topic partition list for callback if needed
Create empty topic partition list for callback if needed

We've seen a case where the offsets pointer in the commit callback was
null. Check for this state and pass an empty topic partiton list into
the callback if it happens.

thijsc/rust-rdkafka,thijsc/rust-rdkafka,fede1024/rust-rdkafka,fede1024/rust-rdkafka,fede1024/rust-rdkafka