lua/utils: export luaL_pushnull and luaL_isnull functions

They are useful in C modules.

Needed for tarantool/tarantool-ee#712

@TarantoolBot document
Title: Update C API reference > Module lua/utils
Product: Tarantool
Root documents: https://www.tarantool.io/en/doc/latest/dev_guide/reference_capi/utils/

The following functions are missed in the documentation:

 * luaL_iscallable
 * luaL_iscdata
 * luaL_isnull
 * luaL_pushnull
 * luaT_call
 * luaT_checktuple
 * luaT_isdecimal
 * luaT_newdecimal
 * luaT_pushdecimal
 * luaT_toibuf
 * luaT_tolstring
 * luaT_tuple_encode
 * luaT_tuple_new

See also: https://github.com/tarantool/doc/issues/2011