Empty section frames cause trouble for the assert added in
e14056e6:

* existing ones may be skipped by MoveBwd(), so a SwTextFrame moves to
  the previous page but the SwSectionFrame that was its mpPrev remains
  on this page; e.g. ooo110854-1.rtf

* a SwSectionFrame moves backward, then some SwTextFrame in it splits
  and the follow tries to move forward, resulting in *new*
  SwSectionFrames on this page that are empty (because the follow is
  eventually joined again); e.g. abi3213-1.rtf

So if there's a section frame, assume it's SNAFU and don't assert.

Change-Id: Ibdca2aa39fae123583e5edf5173f80e8b70ef335
Reviewed-on: https://gerrit.libreoffice.org/73333
Tested-by: Jenkins
Reviewed-by: Michael Stahl <Michael.Stahl@cib.de>