Unifex can't return `nil`

[mickel8]

Consider following spec:

spec get_peer_cert(state) :: payload | (nil :: label)

when compiling with Unifex, there are two issues:

• generated function name does not include nil word i.e.

UNIFEX_TERM get_peer_cert_result_(UnifexEnv *env) {
  return enif_make_atom(env, "");
}

• it returns invalid value i.e.:"" instead of nil