Check if LuaTeX is running

\@empty is defined in LaTeX, so your test fails. Why don't you use ifluatex.sty? It will work with plainTeX too,

How about iftex? it works fine in plain, and gives you some other useful macros.

It is also included in TeXLive.

Here is a more direct answer that uses iftex (see section 3.3):

\usepackage{iftex}
\ifLuaTeX
% ⟨material for LuaTEX⟩
\else
% ⟨material not for LuaTEX⟩
\fi

Related