It wouldn't be terrible to duplicate headers/footers
unnecessarily, but it is terrible to have them disappear.
If the last SectPr has no idea about the section start,
it can't know whether it is continuous or started with
a page break. In that case, just ensure that the
header and footer are explicitly written out.

Change-Id: Iff54ed097b4f8692d7d7764089002b00fbde4f51
Reviewed-on: https://gerrit.libreoffice.org/64821
Tested-by: Jenkins
Reviewed-by: Justin Luth <justin_luth@sil.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
Reviewed-on: https://gerrit.libreoffice.org/65363