Problème : TeX capacity exceeded avec macro et argument optionnel

Je ne comprends pas trop d'où vient le problème suivant.

ECM :

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\documentclass[10pt]{article}
\usepackage{tabularray}
\usepackage{siunitx}
\DeclareSIUnit\noop{\relax}
\begin{document}
\newcommand{\mamacro}[2][]{%
  \if\relax\detokenize{#1}\relax #2 \else #1 \fi & $\unit{\csname #2\endcsname\noop}$}
\begin{tblr}[expand=\mamacro]{lc|lc}
  \mamacro{hecto} & \mamacro[déca]{deca}\\
\end{tblr}
\end{document}
 
 
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