add is_empty function to indicated that no landmarks where generated

Arrangement_on_surface_2/include/CGAL/Arr_point_location/Arr_lm_generator_base.h

@@ -90,6 +90,8 @@ protected:
   bool                     updated;
   int                      num_small_not_updated_changes;
 
+public: 
+  bool is_empty() const { return nn.is_empty(); }
 private:
 
   /*! Copy constructor - not supported. */

Arrangement_on_surface_2/include/CGAL/Arr_point_location/Arr_lm_nearest_neighbor.h

@@ -156,6 +156,9 @@ protected:
   Tree            *m_tree;        // The search tree.
   bool             m_is_empty;    // Is the search tree empty.
 
+public: 
+  bool is_empty() const{ return m_is_empty; }
+  
 private:
 
   /*! Copy constructor - not supported. */