Exception message when Kafka topic is null in FlinkKafkaConsumer

If the topic name is null for the FlinkKafkaConsumer we get an Kafka exception with the message:

Error computing size for field 'topics': Error computing size for field 'name': Missing value for field 'name' which has no default value.

The KafkaTopicDescriptor can check that the topic name empty, blank or null and provide more informative message.