KAFKA-19421: Deflake streams_broker_down_resilience_test

[lucasbru]

streams_broker_down_resilience_test produce messages with null key to a topic with three partitions and expect each partition to be non-empty afterward. But I don't think this is a correct assumption, as a producer may try to be sticky and only produce to two partitions.

This cause occasional flakiness in the test.

The fix is to produce records with keys.