[source-kafka] Describe fails with runtime error if sink is not set

[navidshaikh]

Create a kafka source without sink and try describing it, it fails with runtime error as the nil check for Sink is missing:

Steps to reproduce:

1. Create kafka source

kn source kafka create mykafkasrc --servers my-cluster-kafka-bootstrap.kafka.svc:9092 --topics test-topic --consumergroup test-consumer-group

2. Describe it

kn source kafka describe mykafkasrc
Name:              mykafkasrc
Namespace:         default
Age:               3h
BootstrapServers:  my-cluster-kafka-bootstrap.kafka.svc:9092
Topics:            test-topic
ConsumerGroup:     test-consumer-group

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x134ad18]

goroutine 1 [running]:
knative.dev/client-contrib/plugins/source-kafka/pkg/factories.writeSink(0x184c360, 0xc00042dc50, 0x0)

[...]

Error: exit status 2
Run 'kn --help' for usage

Suggested fix:

Check for nil value here

client-contrib/plugins/source-kafka/pkg/factories/rune_factory.go

Line 189 in 49864fb

writeSink(dw, kafkaSource.Spec.Sink)

[daisy-ycguo]

/assign