Change exception type on addEdge

Blueprints tests require that addEdge(...) calls with a null label
throw IllegalArgumentException.  We used to throw
NullPointerException.

titan-core/src/main/java/com/thinkaurelius/titan/graphdb/blueprints/TitanBlueprintsTransaction.java

@@ -98,6 +98,9 @@ public Edge addEdge(Object id, Vertex outVertex, Vertex inVertex, String label)
 
     @Override
     public TitanEdge addEdge(TitanVertex outVertex, TitanVertex inVertex, String label) {
+        if (null == label) {
+            throw new IllegalArgumentException("Edge label must be non-null");
+        }
         return addEdge(outVertex, inVertex, getEdgeLabel(label));
     }