SvxBrushItem::GetGraphicLink no longer returns a pointer

...since cfb4463d2a "convert
include/editeng/brushitem.hxx from String to OUString", which boldly changed it
from returning a pointer to returning a reference, probably not even taking into
account that that broke the value set (from string+null to just string).

However, that change appears to have worked OK in practice, and there maybe
wasn't a need after all to distinguish a null value from an empty string.  These
two places here were missed in the change, and happened to keep working, as they
picked the operator ==(sal_Unicode const *, OUString const &) overload, that
happens to treat a null first argument like an empty string.

Change-Id: If2d76951fa4bf6c7821321327ad653bd0a95a788

cui/source/tabpages/backgrnd.cxx

@@ -698,7 +698,7 @@ bool SvxBackgroundTabPage::FillItemSet( SfxItemSet* rCoreSet )
 
                 SvxGraphicPosition  eNewPos  = GetGraphicPosition_Impl();
                 const bool          bIsLink  = m_pBtnLink->IsChecked();
-                const bool          bWasLink = (nullptr != rOldItem.GetGraphicLink() );
+                const bool          bWasLink = !rOldItem.GetGraphicLink().isEmpty();
 
 
                 if ( !bIsLink && !bIsGraphicValid )

sw/source/filter/html/css1atr.cxx

@@ -2216,7 +2216,7 @@ static bool OutCSS1_FrameFormatBrush( SwHTMLWriter& rWrt,
     /// output brush of frame format, if its background color is not "no fill"/"auto fill"
     /// or it has a background graphic.
     if( rBrushItem.GetColor() != COL_TRANSPARENT ||
-        nullptr != rBrushItem.GetGraphicLink() ||
+        !rBrushItem.GetGraphicLink().isEmpty() ||
         0 != rBrushItem.GetGraphicPos() )
     {
         OutCSS1_SvxBrush( rWrt, rBrushItem, CSS1_BACKGROUND_FLY, nullptr );