Add failing testcase for #1

[zoni]

This will currently fail with an exception like:

Traceback (most recent call last):
  File "/tmp/venv/foo.py", line 4, in <module>
vault = ot.Vault(Path("/tmp/vault-export")).connect()
  File "/tmp/venv/lib/python3.9/site-packages/obsidiantools/api.py", line 161, in connect
wiki_link_map = self._get_wikilinks_index()
  File "/tmp/venv/lib/python3.9/site-packages/obsidiantools/api.py", line 331, in _get_wikilinks_index
return {k: get_wikilinks(self._dirpath / v)
  File "/tmp/venv/lib/python3.9/site-packages/obsidiantools/api.py", line 331, in <dictcomp>
return {k: get_wikilinks(self._dirpath / v)
  File "/tmp/venv/lib/python3.9/site-packages/obsidiantools/md_utils.py", line 50, in get_wikilinks
plaintext = _get_ascii_plaintext_from_md_file(filepath)
  File "/tmp/venv/lib/python3.9/site-packages/obsidiantools/md_utils.py", line 196, in _get_ascii_plaintext_from_md_file
html = _remove_front_matter(html)
  File "/tmp/venv/lib/python3.9/site-packages/obsidiantools/md_utils.py", line 207, in _remove_front_matter
for fm_detail in hr_content.find_next("p"):
TypeError: 'NoneType' object is not iterable

[mfarragher]

Merging into dev branch