This cleanup should help simplify and clarify immensely.

An empty followStyle is treated the same as not
defining it at all, and if a style specifies a follow, why
not accept any style with that ID, regardless of whether
a UI name was attached?
HOWEVER, both this unit test and
tdf104713_undefinedStyles.docx ignore the w:next
IN MSO. I tested with 2013, 2010, and 2003 and they all
revert back to w:next being the same style
in these broken documents.

Perhaps the existing test of whether sStyleName.isEmpty()
is not the best test, but that is what it was, so keeping
it until some proof document comes along.

Change-Id: I28b1a5d67e6feea6b4d1ffdb77ec27bf6c824251
Reviewed-on: https://gerrit.libreoffice.org/57075
Tested-by: Jenkins
Reviewed-by: Justin Luth <justin_luth@sil.org>