kafka client processConsumerOffsetsMessage panic error

Nobody has claimed this yet.

```
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x38 pc=0xabb7da]

goroutine 14695 [running]:
github.com/linkedin/Burrow/core/internal/consumer.(*KafkaClient).processConsumerOffsetsMessage(0xc0002d6360, 0x0)
/home/titustian/go/src/github.com/linkedin/Burrow/core/internal/consumer/kafka_client.go:234 +0x3a
github.com/linkedin/Burrow/core/internal/consumer.(*KafkaClient).partitionConsumer(0xc0002d6360, 0xe58ca0, 0xc00afd81b0)
/home/titustian/go/src/github.com/linkedin/Burrow/core/internal/consumer/kafka_client.go:171 +0x595
created by github.com/linkedin/Burrow/core/internal/consumer.(*KafkaClient).startKafkaConsumer
/home/titustian/go/src/github.com/linkedin/Burrow/core/internal/consumer/kafka_client.go:226 +0x91a
```
```golang
func (module *KafkaClient) processConsumerOffsetsMessage(msg *sarama.ConsumerMessage) {
logger := module.Log.With(
zap.String("offset_topic", msg.Topic),
zap.Int32("offset_partition", msg.Partition),
zap.Int64("offset_offset", msg.Offset),
)

if len(msg.Value) == 0 {
// Tombstone message - we don't handle them for now
logger.Debug("dropped tombstone")
return
}
```
when the msg is nil ,the logger initialize cause a panic。

Start with core/internal/consumer/kafka_client.go at processConsumerOffsetsMessage around line 234, then inspect its callers at partitionConsumer and startKafkaConsumer. Reproduce or test the nil-message path and verify that processing a nil message no longer panics while the existing tombstone handling remains unchanged.

Written by the indexing model from the issue text.