fix: assorted partition_html() bugs (#2113)
Addresses a cluster of HTML-related bugs:
- empty table is identified as bulleted-table
- `partition_html()` emits empty (no text) tables (#1928)
- `.text_as_html` contains inappropriate `<br>` elements in invalid
locations.
- cells enclosed in `<thead>` and `<tfoot>` elements are dropped (#1928)
- `.text_as_html` contains whitespace padding
Each of these is addressed in a separate commit below.
Fixes #1928.
---------
Co-authored-by: ryannikolaidis <1208590+ryannikolaidis@users.noreply.github.com>
Co-authored-by: scanny <scanny@users.noreply.github.com>
Co-authored-by: Yuming Long <63475068+yuming-long@users.noreply.github.com>
