Resolve FutureWarning

Using of `html_to_docx.create_document_from_html` causes runtime warning:
```
html_to_docx/tag_dispatchers/__init__.py:15: FutureWarning: Truth-testing of elements was a source of confusion and will always return True in future versions. Use specific 'len(elem)' or 'elem is not None' test instead.
  if not element.getparent():
```