MSWordExportBase::OutputSectionBreaks: avoid fake section breaks

Regression from ee9f23bb94b4c2c8c4db6466ecca272a092e9492 (docx export:
invalid sectPr added at the beginning of the doc, 2013-01-10), the
problem was that we even tried to generate section breaks at places
where the two page styles are in practice the same.

Change-Id: Iccf91ce542d9b075b93da25bcd42236db667e3d4