Forward-port to Trunk: NPE when clearing a taxonomy category's name field

When editing the name of an existing taxonomy category, delete all characters. Note that when you delete the last character, you see an NPE log on the console:
Caused by: java.lang.NullPointerException
at org.onehippo.taxonomy.plugin.api.KeyCodec.encode(KeyCodec.java:26)
at org.onehippo.taxonomy.plugin.model.JcrCategoryInfo.setName(JcrCategoryInfo.java:62)
at org.onehippo.taxonomy.plugin.TaxonomyEditorPlugin$NameModel.setObject(TaxonomyEditorPlugin.java:579)
at org.onehippo.taxonomy.plugin.TaxonomyEditorPlugin$NameModel.setObject(TaxonomyEditorPlugin.java:565)
at org.apache.wicket.Component.setDefaultModelObject(Component.java:3052)
at org.apache.wicket.markup.html.form.FormComponent.setModelObject(FormComponent.java:1578)
at org.apache.wicket.markup.html.form.FormComponent.updateModel(FormComponent.java:1104)
at org.apache.wicket.ajax.form.AjaxFormComponentUpdatingBehavior.onEvent(AjaxFormComponentUpdatingBehavior.java:150)
at org.apache.wicket.ajax.AjaxEventBehavior.respond(AjaxEventBehavior.java:146)
at org.apache.wicket.ajax.AbstractDefaultAjaxBehavior.onRequest(AbstractDefaultAjaxBehavior.java:633)
... 36 more