Traceback when formatting an empty directive

[paugier]

Describe the bug

Formatting an empty directive seems broken. For example formatting this

```{graphviz} ../figs/undo-simple.dot

leads to this:

Traceback (most recent call last): File "/home/pierre/.cache/pypoetry/virtualenvs/mercurial-jupyterbook-MIlaqlFz-py3.10/bin/mdformat", line 8, in sys.exit(run()) File "/home/pierre/.cache/pypoetry/virtualenvs/mercurial-jupyterbook-MIlaqlFz-py3.10/lib/python3.10/site-packages/mdformat/main.py", line 8, in run exit_code = mdformat._cli.run(sys.argv[1:]) File "/home/pierre/.cache/pypoetry/virtualenvs/mercurial-jupyterbook-MIlaqlFz-py3.10/lib/python3.10/site-packages/mdformat/_cli.py", line 74, in run formatted_str = mdformat.text( File "/home/pierre/.cache/pypoetry/virtualenvs/mercurial-jupyterbook-MIlaqlFz-py3.10/lib/python3.10/site-packages/mdformat/_api.py", line 37, in text rendering = mdit.render(md) File "/home/pierre/.cache/pypoetry/virtualenvs/mercurial-jupyterbook-MIlaqlFz-py3.10/lib/python3.10/site-packages/markdown_it/main.py", line 275, in render return self.renderer.render(self.parse(src, env), self.options, env) File "/home/pierre/.cache/pypoetry/virtualenvs/mercurial-jupyterbook-MIlaqlFz-py3.10/lib/python3.10/site-packages/mdformat/renderer/init.py", line 57, in render return self.render_tree(tree, options, env, finalize=finalize) File "/home/pierre/.cache/pypoetry/virtualenvs/mercurial-jupyterbook-MIlaqlFz-py3.10/lib/python3.10/site-packages/mdformat/renderer/init.py", line 91, in render_tree text = tree.render(render_context) File "/home/pierre/.cache/pypoetry/virtualenvs/mercurial-jupyterbook-MIlaqlFz-py3.10/lib/python3.10/site-packages/mdformat/renderer/_tree.py", line 11, in render text = renderer(self, context) File "/home/pierre/.cache/pypoetry/virtualenvs/mercurial-jupyterbook-MIlaqlFz-py3.10/lib/python3.10/site-packages/mdformat/renderer/_context.py", line 50, in render_children return separator.join(out for out in render_outputs if out) File "/home/pierre/.cache/pypoetry/virtualenvs/mercurial-jupyterbook-MIlaqlFz-py3.10/lib/python3.10/site-packages/mdformat/renderer/_context.py", line 50, in return separator.join(out for out in render_outputs if out) File "/home/pierre/.cache/pypoetry/virtualenvs/mercurial-jupyterbook-MIlaqlFz-py3.10/lib/python3.10/site-packages/mdformat/renderer/_context.py", line 49, in render_outputs = (child.render(context) for child in node.children) File "/home/pierre/.cache/pypoetry/virtualenvs/mercurial-jupyterbook-MIlaqlFz-py3.10/lib/python3.10/site-packages/mdformat/renderer/_tree.py", line 11, in render text = renderer(self, context) File "/home/pierre/.cache/pypoetry/virtualenvs/mercurial-jupyterbook-MIlaqlFz-py3.10/lib/python3.10/site-packages/mdformat_myst/_directives.py", line 64, in fence code_block = format_directive_content(code_block) File "/home/pierre/.cache/pypoetry/virtualenvs/mercurial-jupyterbook-MIlaqlFz-py3.10/lib/python3.10/site-packages/mdformat_myst/_directives.py", line 75, in format_directive_content parse_result = parse_opts_and_content(raw_content) File "/home/pierre/.cache/pypoetry/virtualenvs/mercurial-jupyterbook-MIlaqlFz-py3.10/lib/python3.10/site-packages/mdformat_myst/_directives.py", line 104, in parse_opts_and_content line = lines.pop(0) IndexError: pop from empty list

### Reproduce the bug

Format a file with an empty directive...

### List your environment

jupyter-book --version Jupyter Book : 0.13.2 External ToC : 0.3.1 MyST-Parser : 0.15.2 MyST-NB : 0.13.2 Sphinx Book Theme : 0.3.3 Jupyter-Cache : 0.4.3 NbClient : 0.5.13