do not paint background if no fade color was set

The condition got lost in commit 2e8b2e6d .

Change-Id: I8c9a6baf56dfe36d55e02476823e2847d77a7f58

slideshow/source/engine/transitions/slidetransitionfactory.cxx

@@ -420,11 +420,14 @@ void FadingSlideChange::prepareForRun(
     const ViewEntry& rViewEntry,
     const cppcanvas::CanvasSharedPtr& rDestinationCanvas )
 {
-    // clear page to given fade color. 'Leaving' slide is
-    // painted atop of that, but slowly fading out.
-    fillPage( rDestinationCanvas,
-              ::basegfx::B2DSize( getEnteringSlideSizePixel( rViewEntry.mpView ) ),
-              *maFadeColor );
+    if ( maFadeColor )
+    {
+        // clear page to given fade color. 'Leaving' slide is
+        // painted atop of that, but slowly fading out.
+        fillPage( rDestinationCanvas,
+                ::basegfx::B2DSize( getEnteringSlideSizePixel( rViewEntry.mpView ) ),
+                *maFadeColor );
+    }
 }
 
 void FadingSlideChange::performIn(