DNS batch getZone do not return null for NOT FOUND

Although documented [here](https://github.com/GoogleCloudPlatform/gcloud-java/blob/master/gcloud-java-dns/src/main/java/com/google/cloud/dns/DnsBatch.java#L116) 404 (NOT FOUND) exceptions are not translated into `null` (should be done [here](https://github.com/GoogleCloudPlatform/gcloud-java/blob/master/gcloud-java-dns/src/main/java/com/google/cloud/dns/DnsBatch.java#L271)).
