[API] Make hb_face_reference_table() return empty blob instead of NULL

The idea here is that:

  - Like pretty much all other API in harfbuzz, user does not have to
    check for NULL.

  - In any caller code, the case of missing table should be handled
    exactly the same way that a too-short table is handled.  Turning
    a non-existent talbe into a table of size 0 makes the user code
    safer.

src/hb-font.cc

@@ -417,6 +417,8 @@ hb_face_reference_table (hb_face_t *face,
     return &_hb_blob_nil;
 
   blob = face->get_table (tag, face->user_data);
+  if (unlikely (!blob))
+    blob = hb_blob_create_empty();
 
   return blob;
 }

src/hb-open-type-private.hh

@@ -286,9 +286,6 @@ struct Sanitizer
 
     /* TODO is_sane() stuff */
 
-    if (!blob)
-      return hb_blob_create_empty ();
-
   retry:
     (void) (HB_DEBUG_SANITIZE &&
       fprintf (stderr, "Sanitizer %p start %s\n", blob, HB_FUNC));