Avoid brace stripping in \__tblr_extract_table_commands:N

[muzimuzhi]

Follow-up to 5d380e6 (feat: turn off step tracing by default, 2024-04-05). Found when preparing example used in lvjr/tabularray#501 (comment).

\__tblr_extract_table_commands:N converts {b} to b and {b}b to bb. The braces are stripped by use of

\exp_last_unbraced:NV \__tblr_extract_table_commands_next:n #1 \q_stop

\documentclass{article}
\usepackage{tabularray}

\begin{document}
\SetTblrTracing{+text}

\begin{tblr}{}
  a & \\
  {b} & {b}b \\
  {{c}} \\
\end{tblr}
\end{document}

> The spec list text_1 contains the pairs:.
>  {[1][1]}  =>  {a}.
>  {[1][2]}  =>  {}.
>  {[2][1]}  =>  {b}.
>  {[2][2]}  =>  {bb}.
>  {[3][1]}  =>  {c}.
>  {[4][1]}  =>  {}.

tabularray-dev/tabularray/tabularray.sty

Lines 2828 to 2840 in 1bcb706

	\cs_new_protected:Npn \__tblr_extract_table_commands:N #1
	{
	\tl_clear:N \l__tblr_saved_table_commands_before_cell_text_tl
	\tl_clear:N \l__tblr_saved_cell_text_after_table_commands_tl
	\exp_last_unbraced:NV \__tblr_extract_table_commands_next:n #1 \q_stop
	\tl_if_empty:NF \l__tblr_saved_table_commands_before_cell_text_tl
	{
	\__tblr_prop_gput:neV { command }
	{[\int_use:N \c@rownum][\int_use:N \c@colnum]}
	\l__tblr_saved_table_commands_before_cell_text_tl
	}
	\tl_set_eq:NN #1 \l__tblr_saved_cell_text_after_table_commands_tl
	}