Return nothing for outlineRowForPageIndex: when there is no outline, as the outlineView may perhaps be stale when the document is reloading.
Return nothing for outlineRowForPageIndex: when there is no outline, as the outlineView may perhaps be stale when the document is reloading.

git-svn-id: 765db44a9620ed55f4f8addfd3c14fe8cd57f6bb@2713 0fceea05-a30d-0410-8a8b-80ef821fb0a1

nanoant/Skim,nanoant/Skim,nanoant/Skim,nanoant/Skim,nanoant/Skim