DOCX import: fix additional empty paragraphs around footnote text

Regression from commit abe1e852a232e3bdab7b9177d099fe3929f5a4aa (handle
properly page breaks even if a page contains only a frame (fdo#55381),
2014-04-21), DomainMapper::lcl_endSectionGroup() inserted an empty
paragraph before and after the real footnote text. Disable addition of
the dummy paragraph for footnotes.

Change-Id: I6e1954c9bd5520ac284d66244ce1c9004e970305