CSSStyleDeclaraction returns empty string

LayoutTests/css3/blending/script-tests/blend-mode-property-parsing-invalid.js

 description("Test the parsing of the mix-blend-mode 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;
 stylesheet.insertRule("body { mix-blend-mode: multiply; }", 0);
 
 function testInvalidFilterRule(description, rule)
 {
     debug("");
     debug(description + " : " + rule);
 
     stylesheet.insertRule("body { mix-blend-mode: " + rule + "; }", 0);
     cssRule = stylesheet.cssRules.item(0);
 
     shouldBe("cssRule.type", "1");
 
     declaration = cssRule.style;
     shouldBe("declaration.length", "0");
-    shouldBe("declaration.getPropertyValue('mix-blend-mode')", "null");
+    shouldBeEqualToString("declaration.getPropertyValue('mix-blend-mode')", "");
 }
 
 testInvalidFilterRule("Too many parameters", "overlay overlay");
 testInvalidFilterRule("Wrong type", "\"5px\"");
 testInvalidFilterRule("Trailing comma", "overlay,");
 
 
 successfullyParsed = true;