[API] Add hb_*_get_empty() for all objects

src/hb-font.cc

diff --git a/src/hb-font.cc b/src/hb-font.cc
index 6cd436d..bb8f84c 100644
--- a/src/hb-font.cc
+++ b/src/hb-font.cc
@@ -160,6 +160,12 @@
 }
 
 hb_font_funcs_t *
+hb_font_funcs_get_empty (void)
+{
+  return &_hb_font_funcs_nil;
+}
+
+hb_font_funcs_t *
 hb_font_funcs_reference (hb_font_funcs_t *ffuncs)
 {
   return hb_object_reference (ffuncs);
@@ -404,6 +410,12 @@
 				    (hb_destroy_func_t) _hb_face_for_data_closure_destroy);
 }
 
+hb_face_t *
+hb_face_get_empty (void)
+{
+  return &_hb_face_nil;
+}
+
 
 hb_face_t *
 hb_face_reference (hb_face_t *face)
@@ -534,6 +546,12 @@
 }
 
 hb_font_t *
+hb_font_get_empty (void)
+{
+  return &_hb_font_nil;
+}
+
+hb_font_t *
 hb_font_reference (hb_font_t *font)
 {
   return hb_object_reference (font);