DOCX Input: Fix some text being underlined incorrectly

The "none" underline style was being translated incorrectly.
Fixes #1189311 [Private bug](https://bugs.launchpad.net/calibre/+bug/1189311)
DOCX Input: Fix empty paragraphs not being rendered
DOCX Input: Fix some manual page breaks being ignored

Fixes #1188573 [Private bug](https://bugs.launchpad.net/calibre/+bug/1188573)