This was a regression introduced with 2af9040d
"Cleaned up utl::UCBContentHelper."  GetSize exhibited the same problem and has
also been fixed.  IsYounger exhibits a similar problem, but has not been
addressed, as it is unclear what to return in case no dates can be compared; it
is only used in one place (handling SID_EDITDOC in
SfxViewFrame::ExecReload_Impl, sfx2/source/view/viewfrm.cxx) where, it appears,
only contents for which DateModified /does/ yield a non-void value are relevant.

Change-Id: Iff9f5e1ed6f45eede117713d64a265b98f6862b1