Fix kafka input binding looping when not topics are set (#1412)

* binding kafka: check if topics are not empty before reading

Signed-off-by: Tim Burkert <burkert.tim@gmail.com>

* binding kafka: log warning instead of returning error

Signed-off-by: Tim Burkert <burkert.tim@gmail.com>

Co-authored-by: Yaron Schneider <schneider.yaron@live.com>
Co-authored-by: Artur Souza <artursouza.ms@outlook.com>
Co-authored-by: Dapr Bot <56698301+dapr-bot@users.noreply.github.com>

bindings/kafka/kafka.go

@@ -226,6 +226,11 @@ func (k *Kafka) getSyncProducer(meta *kafkaMetadata) (sarama.SyncProducer, error
 }
 
 func (k *Kafka) Read(handler func(*bindings.ReadResponse) ([]byte, error)) error {
+	if len(k.topics) == 0 {
+		k.logger.Warnf("kafka binding: no topic defined, input bindings will not be started")
+		return nil
+	}
+
 	config := sarama.NewConfig()
 	config.Version = sarama.V1_0_0_0
 	config.Consumer.Offsets.Initial = k.initialOffset