[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