lua, PanelLayout - don't create a div if preamble is empty

[cderv]

closes #11964

Default preamble is empty list and we need to make sure to not create a Div when empty preamble is found.

In typst or HTML this would create an empty block - for LaTeX we do have a finalizer to descaffold a Div with no attributes at all.

[cscheid]

This is a good immediate fix, thank you!

We might consider adding the descaffolding filter to typst. But we need to be careful: in typst these divs are important. Specifically, they introduce a new lexical scope environment, so that changing values inside the scope doesn't propagate outside it.

[cderv]

Thanks !

We might consider adding the descaffolding filter to typst. But we need to be careful

Yeah I was planning to leave this on the side. We shouldn't be needing this in theory if we don't create those empty divs. In LaTeX, I think we added this specific handling in finalizer because it could break the document making it not renderable.