htmlvis: can't use only "annis:BEGIN" or "annis:END"

[thomaskrause]

If there are only BEGIN or END statements in the html visualization configuration an exception is thrown:

Caused by: java.util.NoSuchElementException: null
    at java.util.TreeMap.key(TreeMap.java:1323) ~[na:1.8.0_66-internal]
    at java.util.TreeMap.firstKey(TreeMap.java:290) ~[na:1.8.0_66-internal]
    at annis.visualizers.htmlvis.HTMLVis.createHTML(HTMLVis.java:395) ~[classes/:na]
    at annis.visualizers.htmlvis.HTMLVis.createComponent(HTMLVis.java:129) ~[classes/:na]
    at annis.visualizers.htmlvis.HTMLVis.createComponent(HTMLVis.java:73) ~[classes/:na]
    at annis.gui.resultview.VisualizerPanel.createComponent(VisualizerPanel.java:292) ~[classes/:na]
    at annis.gui.resultview.VisualizerPanel.access$500(VisualizerPanel.java:86) ~[classes/:na]
    at annis.gui.resultview.VisualizerPanel$LoadComponentTask.call(VisualizerPanel.java:621) ~[classes/:na]
    at annis.gui.resultview.VisualizerPanel$LoadComponentTask.call(VisualizerPanel.java:612) ~[classes/:na]