[SPARK-7466] DAG visualization: fix orphan nodes

Simple fix. We were comparing an option with `null`.

Before:
<img src="https://issues.apache.org/jira/secure/attachment/12731383/before.png" width="250px"/>
After:
<img src="https://issues.apache.org/jira/secure/attachment/12731384/after.png" width="250px"/>

Author: Andrew Or <andrew@databricks.com>

Closes #6002 from andrewor14/dag-viz-orphan-nodes and squashes the following commits:

a1468dc [Andrew Or] Fix null check

core/src/main/scala/org/apache/spark/ui/scope/RDDOperationGraph.scala

@@ -101,7 +101,7 @@ private[ui] object RDDOperationGraph extends Logging {
       val node = nodes.getOrElseUpdate(
         rdd.id, RDDOperationNode(rdd.id, rdd.name, rdd.storageLevel != StorageLevel.NONE))
 
-      if (rdd.scope == null) {
+      if (rdd.scope.isEmpty) {
         // This RDD has no encompassing scope, so we put it directly in the root cluster
         // This should happen only if an RDD is instantiated outside of a public RDD API
         rootCluster.attachChildNode(node)