handle properly page breaks even if a page contains only a frame (fdo#55381)

The test document has 4 pages which only contain frames, first 3 pages
(sections) have <p> block with <framePr> and another <p> with just <sectPr>,
and the second paragraph would be removed, thus there would be no nodes on which
to actually set the page style and they would be set on nodes for the previous
page, overwriting its page style. The last page does not have its own <p>
at all, so it needs to be faked.

The changes in finishParagraph() are because of e.g. the comments-nested.odt
sw test, which there causes exception because of unknown property ParaStyleName
(comments do not use those it seems) and the code is skipped over (which
is probably a bug on its own, but it's unrelated to this change otherwise).

Change-Id: I9d37f992407a9b649c710d2a031ec4447fb11771
Reviewed-on: https://gerrit.libreoffice.org/9115
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Miklos Vajna <vmiklos@collabora.co.uk>