Null records breaks librdkafka

Pranav Rathi ran the librdkafka tests with the 4.0 branch and identified an issue with empty fetch responses:
 
"Before 4.0, we were receiving 0 when there were no records. It meant empty records list I think. Now in 4.0, we are receiving -1. Librdkadka treat this -1 as invalid and raises parse error."