Collection without keywords results in HTTP 500

[WillemElbers]

Keywords are optional, but leaving this field empty results in a nullpointer exception causing a HTTP 500:

Caused by: java.lang.NullPointerException
	at java.util.StringTokenizer.<init>(StringTokenizer.java:199) ~[?:?]
	at java.util.StringTokenizer.<init>(StringTokenizer.java:221) ~[?:?]
	at eu.clarin.cmdi.virtualcollectionregistry.gui.pages.crud.v2.editor.CreateAndEditPanel.persist(CreateAndEditPanel.java:524) ~[classes/:?]
	at eu.clarin.cmdi.virtualcollectionregistry.gui.pages.crud.v2.editor.CreateAndEditPanel.access$700(CreateAndEditPanel.java:37) ~[classes/:?]
	at eu.clarin.cmdi.virtualcollectionregistry.gui.pages.crud.v2.editor.CreateAndEditPanel$5.onClick(CreateAndEditPanel.java:279) ~[classes/:?]