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

[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.