Suppress 'underfull {h,v}box' warning for 'empty' boxes, as original [Lua]TeX.

Suppress 'underfull {h,v}box' warning for 'empty' boxes, as original [Lua]TeX.

 Note that this code only works for LuaTeX rev 5023 or later.

 LuaTeX and LuaJITTeX which is shipped with TL14 may causes
 'assertion failed' error with this code.