If .attr() is run on no elements undefined should be returned (as should be...

If .attr() is run on no elements undefined should be returned (as should be the case when no attribute is found). Fixes #6012.

src/core.js

@@ -799,7 +799,7 @@ function access( elems, key, value, exec, fn, pass ) {
 	}
 	
 	// Getting an attribute
-	return length ? fn( elems[0], key ) : null;
+	return length ? fn( elems[0], key ) : undefined;
 }
 
 function now() {

test/unit/attributes.js

@@ -4,7 +4,7 @@ var bareObj = function(value) { return value; };
 var functionReturningObj = function(value) { return (function() { return value; }); };
 
 test("attr(String)", function() {
-	expect(28);
+	expect(30);
 
 	// This one sometimes fails randomly ?!
 	equals( jQuery('#text1').attr('value'), "Test", 'Check for value attribute' );
@@ -61,6 +61,9 @@ test("attr(String)", function() {
 	select.appendChild( optgroup );
 
 	equals( jQuery(option).attr("selected"), true, "Make sure that a single option is selected, even when in an optgroup." );
+
+	ok( jQuery("<div/>").attr("doesntexist") === undefined, "Make sure undefined is returned when no attribute is found." );
+	ok( jQuery().attr("doesntexist") === undefined, "Make sure undefined is returned when no element is there." );
 });
 
 if ( !isLocal ) {