Out-of-bound span is not settable #419

\documentclass{article}
\usepackage{tabularray}

\begin{document}
\begin{tblr}{
  cell{2}{2}={c=2,r=2}{}
}
  1 & 2 \\
  3 & 4 \\
\end{tblr}
\end{document}

throws several errors, among which the first is

> Step: build the whole table.

! Missing number, treated as zero.
<to be read again> 
                   \__dim_eval_end: 
l.10 \end
         {tblr}
A number should have been here; I inserted `0'.
(If you can't figure out why I needed to see a number,
look up `weird error' in the index to The TeXbook.)

Like #418, such span setting should be dropped/cropped with an appropriate warning message, instead of the current low level error. Warning is preferable because such setting may first be set globally by \SetTblrInner, then dropped/cropped by a following small-sized tblr tabular.