Refined fix for #9633

Refined fix for #9633

A branch inset modifies the layout of the internal structures in
which the text is organized. When a branch is active, it is as if it
was not there, but its only presence makes a paragraph which would not
be the last one to actually be the last one, or the check for the
language of the previous paragraph to fail because there is no
previous paragraph before the first one in a branch inset.
Oney way I found to tackle it, is tracking whether the typesetted
paragraphs are actually part of an active branch inset and acting
accordingly.