Editor will produce an invalid SMIL file if user tries to delete whole video

[JamesUoM]

To Reproduce:

Go to stable.opencast.org edit the dual-stream-demo

1.Select the default segment and mark it for deletion
2. Save and process
3. The workflow will fail on the editor WOH with:

org.opencastproject.workflow.api.WorkflowOperationException: org.opencastproject.smil.api.SmilException: Smil does not define any elements
	at org.opencastproject.workflow.handler.videoeditor.VideoEditorWorkflowOperationHandler.resume(VideoEditorWorkflowOperationHandler.java:470)
	at org.opencastproject.workflow.handler.videoeditor.VideoEditorWorkflowOperationHandler.start(VideoEditorWorkflowOperationHandler.java:31

Expected/Desired behaviour

The editor should not allow the user remove the whole video. Instead it should inform the user to delete video via the Opencast Admin UI or simply leave it unpublished.