Tests: Update domEqual() to work with jQuery git

jQuery now returns `null` for empty attributes instead of `undefined`.

Ref gh-1516

(cherry picked from commit e4363ab)

Author: scottgonzalez

tests/unit/testsuite.js

@@ -301,11 +301,11 @@ window.domEqual = function( selector, modifier, message ) {
 			result = {};
 		$.each( properties, function( index, attr ) {
 			var value = elem.prop( attr );
-			result[ attr ] = value !== undefined ? value : "";
+			result[ attr ] = value != null ? value : "";
 		});
 		$.each( attributes, function( index, attr ) {
 			var value = elem.attr( attr );
-			result[ attr ] = value !== undefined ? value : "";
+			result[ attr ] = value != null ? value : "";
 		});
 		result.style = getElementStyles( elem[ 0 ] );
 		result.events = $._data( elem[ 0 ], "events" );