Fix refs positionning using a div for PDF output

[cderv]

This should fix #6867

As discussed, this is a regression in 1.4 due to some change at #6620 regarding flattening the divs in coalesce_raw() at src\resources\filters\quarto-finalize\coalesceraw.lua

This PR complements the fix at #6952 by also checking for empty ids on the Div before doing the flattening.

@cscheid is that too wide ? We could just disallow flattening for div with id refs if needed. I am not too sure why we would flatten in LaTeX all the div of the document - but you know better.

Also I added some tests. For that, I thought about your Lua testing tricks. But it required to add and entry point at the very end .
Is that ok ?

Otherwise, classic test is to check inside output.

To prevent regression, I also added some tests about the refs positioning feature for natbib and biblatex, which is a Quarto feature (https://quarto.org/docs/authoring/footnotes-and-citations.html#bibliography-generation) cc @dragonstyle I did not found any for this feature

[cscheid]

I am not too sure why we would flatten in LaTeX all the div of the document - but you know better.

One of the reasons is that divs are useful to organize the AST internally, but they are "useless" in LaTeX, except that they introduce a line break (because they're blocks), and often we don't want extra line breaks in LaTeX.

[cderv]

except that they introduce a line break (because they're blocks)

That was my missing point ! Good to know !