A DOCX document that has no sections can still contain one or more
tables. Such files are never created by Word but Word can open them just
fine. These can be and are generated by other applications.
Use the newly-added `Document.iter_inner_content()` method added
upstream in `python-docx` to capture both paragraphs and tables from a
section-less DOCX document.
This generalizes the fix for MS Teams chat-transcripts (an example of
sectionless-docx) implemented in #1825.
This PR resolves#1816
- current docx partition assumes all contents are in sections
- this is not true for MS Teams chat transcript exported to docx
- now the code checks if there are sections or not; if not then iterate
through the paragraphs and partition contents in the paragraphs
