\l__csl_language_code_map_prop is not initiated corrected

[josephwright]

Describe the bug

\l__csl_language_code_map_prop is not initiated correctly, leading to an error with the latest expl3 release. See e.g. https://tex.stackexchange.com/questions/709871/failed-to-compile-latex-document-citation-style-language

Additional information

• TeX distribution: TL'23
• Package citation-style-language version: {2023-10-05} {0.4.5}
• LaTeX engine: all

To Reproduce

\documentclass{article}
\usepackage{citation-style-language}

\begin{document}
\end{document}

[zepinglee]

Thanks for your report and fix!