Loader fails for null groups

CX reader cannot read this CX file:

https://gist.github.com/keiono/bced7d92138e05838b742e8c048fea75

It fails with the following exception:

```
java.lang.NullPointerException
at org.cytoscape.io.internal.cx_reader.CxToCy.addGroups(CxToCy.java:487)
at org.cytoscape.io.internal.cx_reader.CxToCy.createNetwork(CxToCy.java:460)
at org.cytoscape.io.internal.cx_reader.CytoscapeCxNetworkReader.run(CytoscapeCxNetworkReader.java:214)
at org.cytoscape.work.internal.task.JDialogTaskManager$TaskRunnable.innerRun(JDialogTaskManager.java:342)
at org.cytoscape.work.internal.task.JDialogTaskManager$TaskRunnable.run(JDialogTaskManager.java:355)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
```