...but only with old MSWord compat flag SplitPgBreakAndParaMark.

All of the other cases (COLUMN_BREAK and non-empty runs) split
the paragraph, so why not here?  This document shows it is needed,
but only for SplitPgBreakAndParaMark documents.

Note: Word 2003 doesn't display "modern" docx well in this regard.
It adds extra paragraphs where it shouldn't. There are already
example unit tests that ensure that extra paragraphs aren't written
for SplitPgBreakAndParaMark == false.

The actual bug's document is not related to the compatibility flag.
That will be handled in separate commit.

Change-Id: I27399780c909663f9a2b21974a5b385bea67f9ec
Reviewed-on: https://gerrit.libreoffice.org/70835
Tested-by: Jenkins
Reviewed-by: Justin Luth <justin_luth@sil.org>