Separate nil font-funcs from parent-peeking font-funcs

API change: all fonts created with hb_font_create() now inherit from
(ie have parent) hb_font_get_empty().
diff --git a/test/api/test-font.c b/test/api/test-font.c
index ef5b08f..34f6c74 100644
--- a/test/api/test-font.c
+++ b/test/api/test-font.c
@@ -386,7 +386,7 @@
 
 
   g_assert (hb_font_get_face (font) == face);
-  g_assert (hb_font_get_parent (font) == NULL);
+  g_assert (hb_font_get_parent (font) == hb_font_get_empty ());
   subfont = hb_font_create_sub_font (font);
   g_assert (hb_font_get_parent (subfont) == font);
   hb_font_set_parent(subfont, NULL);