Allow styling of class/id that do not exist in the table

[mariuslp]

Currently the compilation fails if we try to apply style to a cell class/id that does not exist.

\documentclass{article}
\usepackage{tabularray}
\usepackage{xcolor}

\begin{document}

\begin{table}
\begin{tblr}{
    colspec=lll,
    cell{exists} = {green9},
    % cell{notexists} = {red9},  % will not compile
}
       \SetChild{class=exists} 1 & 2 & 3 \\
\end{tblr}
\end{table}
\end{document}

A better approach would be to ignore the styling, but emit a warning that no cell is using this class/id (e.g. in the example above, a warning that no cell has the style notexists).
I am willing to look into this but I am very new to Latex3 and some pointers about where to look in the source code would be appreciated.