CSSStyleDeclaraction returns empty string

LayoutTests/css3/filters/script-tests/filter-property-parsing-invalid.js

 description("Test the parsing of the -webkit-filter property.");
 
 // These have to be global for the test helpers to see them.
 var stylesheet, cssRule, declaration;
 var styleElement = document.createElement("style");
 document.head.appendChild(styleElement);
 stylesheet = styleElement.sheet;
 
 function testInvalidFilterRule(description, rule)
 {
     debug("");
     debug(description + " : " + rule);
 
     stylesheet.insertRule("body { -webkit-filter: " + rule + "; }", 0);
     cssRule = stylesheet.cssRules.item(0);
   
     shouldBe("cssRule.type", "1");
 
     declaration = cssRule.style;
     shouldBe("declaration.length", "0");
-    shouldBe("declaration.getPropertyValue('-webkit-filter')", "null");
+    shouldBeEqualToString("declaration.getPropertyValue('-webkit-filter')", "");
 }
 
 testInvalidFilterRule("Too many parameters", "url(#a #b)");
 
 testInvalidFilterRule("Length instead of number", "grayscale(10px)");
 testInvalidFilterRule("Too many parameters", "grayscale(0.5 0.5)");
 testInvalidFilterRule("Too many parameters and commas", "grayscale(0.5, 0.5)");
 testInvalidFilterRule("Trailing comma", "grayscale(0.5,)");
 testInvalidFilterRule("Negative parameter", "grayscale(-0.5)");
 testInvalidFilterRule("Negative percent", "grayscale(-10%)");
@@ skipping 59 matching lines @@
 testInvalidFilterRule("Not enough lengths", "drop-shadow(red 1px)");
 testInvalidFilterRule("Missing units", "drop-shadow(red 1 2 3)");
 testInvalidFilterRule("Inset", "drop-shadow(red 1px 2px 3px inset)");
 testInvalidFilterRule("Too many parameters", "drop-shadow(red 1px 2px 3px 4px)");
 testInvalidFilterRule("Commas", "drop-shadow(red, 1px, 2px, 3px)");
 testInvalidFilterRule("Negative radius", "drop-shadow(10px 10px -1px red)");
 
 testInvalidFilterRule("Unknown function", "fancify(150%)");
 
 successfullyParsed = true;