fix(latex-renderer): thematic break on new line

[choeppler]

Given the following markdown snippet

line

---

other line

and creating a pdf from the rendered LaTeX with pdflatex, version pdfTeX 3.141592653-2.6-1.40.25 (TeX Live 2023/Debian)), I get

However, I'd expect the horizontal rule to be on a separate line as shown in the following image

The attached patch fixes that by adding an empty line before the \hrulefill.

To reproduce, run the following script and create a pdf from the resulting LaTeX-file via pdflatex test.tex:

#!/bin/env python3

import mistletoe
from mistletoe.latex_renderer import LaTeXRenderer

def render(f):
   with open(f, 'r') as fp:
     return mistletoe.markdown(fp, LaTeXRenderer)

test = """
line

---

other line
"""

open('test.md', 'w').write(test)
open('test.tex', 'w').write(render('test.md'))

[coveralls]

coverage: 94.207%. remained the same
when pulling 4697d0a on boschresearch:fix-latex-render-thematic-break
into 0b7cdf7 on miyuchina:master.

[pbodnar]

Seems to make sense, thank you, @choeppler! 👍