Add repeat_header Support for xltabular

[amarakon]

In my fork of knitr, I added support for the xltabular environment for the kable function. The repeat_header LaTeX option only works with xltabular if there is a caption. Otherwise, it gives me the following error:

! Missing number, treated as zero.
<to be read again>
                   N
l.357 \end{xltabular}

This line is wrong in the LaTeX output:

\multicolumn{NA}{@{}l}{\textit{(continued)}}\\

It should be like this instead:

\multicolumn{4}{l}{\textit{(continued)}}\\