Handle null in value list in getLayeredShorthandValue

LayoutTests/fast/css/script-tests/image-set-setting.js

Index: LayoutTests/fast/css/script-tests/image-set-setting.js
diff --git a/LayoutTests/fast/css/script-tests/image-set-setting.js b/LayoutTests/fast/css/script-tests/image-set-setting.js
index 734c9ad11ac13adf61f6db21b71fe97c063d6c08..1488e25648559cc080e301dbd93909b16141463a 100644
--- a/LayoutTests/fast/css/script-tests/image-set-setting.js
+++ b/LayoutTests/fast/css/script-tests/image-set-setting.js
@@ -5,7 +5,7 @@ function testComputedStyle(property, fullRule)
     var div = document.createElement("div");
     document.body.appendChild(div);
     div.setAttribute("style", property + ": " + fullRule);
-    var computedValue = div.style.background;
+    var computedValue = div.style.backgroundImage;
     document.body.removeChild(div);
     return computedValue;
 }