Change hb-ft nullptr scheme

[behdad]

So instead of what we did in #2502 and #2494, I believe the correct way is to NOT call hb_font_set_funcs in _hb_ft_font_set_funcs if _hb_ft_font_create failed. That is, there's no point getting into the callbacks with a nullptr. Right now all callbacks need to protect against it, and the issues we fixed just did that for two that were discovered...