Simplify AbstractTask constructor

indexing-service/src/main/java/com/metamx/druid/indexing/common/task/AbstractTask.java

@@ -58,11 +58,7 @@ public abstract class AbstractTask implements Task
 
   protected AbstractTask(String id, String groupId, String dataSource, Interval interval)
   {
-    this.id = Preconditions.checkNotNull(id, "id");
-    this.groupId = Preconditions.checkNotNull(groupId, "groupId");
-    this.taskResource = new TaskResource(id, 1);
-    this.dataSource = Preconditions.checkNotNull(dataSource, "dataSource");
-    this.interval = Optional.fromNullable(interval);
+    this(id, groupId, new TaskResource(id, 1), dataSource, interval);
   }
 
   protected AbstractTask(String id, String groupId, TaskResource taskResource, String dataSource, Interval interval)