SetNullVlaueTitle(fullFieldName, translatedTextToReplace);

Replaces the empty or null text for the specified tree field to the specified content.

Call this during the afterViewLoad event:

afterViewLoad:function(){
this.departmentTree.setNullValueTitle("dv.dv_id",
"Unassigned Division");
}

View: http://localhost:8080/archibus/schema/ab-products/solutions/parts/tree/ab-ex-setnullvalue-tree.axvw

View: http://localhost:8080/archibus//schema/ab-products/solutions/parts/tree/ab-ex-setnullvalue-tabletree.axvw