[ML] If snapshot list is empty, return after signaling failure

Original commit: elastic/x-pack-elasticsearch@a89f88da11

plugin/src/main/java/org/elasticsearch/xpack/ml/action/DeleteModelSnapshotAction.java

@@ -164,6 +164,7 @@ public class DeleteModelSnapshotAction extends Action<DeleteModelSnapshotAction.
                         if (deleteCandidates.isEmpty()) {
                             listener.onFailure(new ResourceNotFoundException(Messages.getMessage(Messages.REST_NO_SUCH_MODEL_SNAPSHOT,
                                     request.getSnapshotId(), request.getJobId())));
+                            return;
                         }
                         ModelSnapshot deleteCandidate = deleteCandidates.get(0);