Empty MyST files fail to open as notebooks (IndexError: list index out of range)

[mwouts]

This can be seen with the test test_read_empty_text_notebook at https://github.com/mwouts/jupytext/tree/test_read_empty_text_notebook

The error is the following:

tests/test_read_empty_text_notebook.py:8 (test_read_empty_text_notebook[.mnb])
ext = '.mnb'
tmp_path = PosixPath('/tmp/pytest-of-marc/pytest-6/test_read_empty_text_notebook_34')

    @pytest.mark.parametrize('ext', set(NOTEBOOK_EXTENSIONS)-{'.ipynb'})
    def test_read_empty_text_notebook(ext, tmp_path):
        if ext == '.qmd' and not is_quarto_available(min_version="0.2.0"):
            pytest.skip("quarto is not available")
        if ext in myst_extensions(no_md=True) and not is_myst_available():
            pytest.skip('MyST is not available')
    
        empty_nb = (tmp_path / "notebook").with_suffix(ext)
        empty_nb.touch()
    
>       nb = jupytext.read(empty_nb)

tests/test_read_empty_text_notebook.py:19: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
jupytext/jupytext.py:409: in read
    return read(stream, as_version=as_version, fmt=fmt, config=config, **kwargs)
jupytext/jupytext.py:418: in read
    return reads(fp.read(), fmt, config=config, **kwargs)
jupytext/jupytext.py:370: in reads
    notebook = reader.reads(text, **kwargs)
jupytext/jupytext.py:104: in reads
    return myst_to_notebook(s)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

text = '', code_directive = '{code-cell}', raw_directive = '{raw-cell}'
add_source_map = False

    def myst_to_notebook(
        text,
        code_directive=CODE_DIRECTIVE,
        raw_directive=RAW_DIRECTIVE,
        add_source_map=False,
    ):
        """Convert text written in the myst format to a notebook.
    
        :param text: the file text
        :param code_directive: the name of the directive to search for containing code cells
        :param raw_directive: the name of the directive to search for containing raw cells
        :param add_source_map: add a `source_map` key to the notebook metadata,
            which is a list of the starting source line number for each cell.
    
        :raises MystMetadataParsingError if the metadata block is not valid JSON/YAML
    
        NOTE: we assume here that all of these directives are at the top-level,
        i.e. not nested in other directives.
        """
        raise_if_myst_is_not_available()
    
        tokens = get_parser().parse(text + "\n")
        lines = text.splitlines()
        md_start_line = 0
    
        # get the document metadata
        metadata_nb = {}
>       if tokens[0].type == "front_matter":
E       IndexError: list index out of range

jupytext/myst.py:286: IndexError