Make sure that undefined is always returned for undefined data proper…

…ties. Fixes #6166.

src/data.js

@@ -25,7 +25,7 @@ jQuery.extend({
 		var id = elem[ expando ], cache = jQuery.cache, thisCache;
 
 		if ( !id && typeof name === "string" && data === undefined ) {
-			return null;
+			return;
 		}
 
 		// Compute a unique ID for the element

test/unit/data.js

@@ -24,12 +24,15 @@ test("expando", function(){
 });
 
 test("jQuery.data", function() {
-	expect(8);
-	var div = jQuery("#foo")[0];
-	equals( jQuery.data(div, "test"), undefined, "Check for no data exists" );
+	expect(9);
+	var div = document.createElement("div");
+
+	ok( jQuery.data(div, "test") === undefined, "Check for no data exists" );
 
 	jQuery.data(div, "test", "success");
 	equals( jQuery.data(div, "test"), "success", "Check for added data" );
+
+	ok( jQuery.data(div, "notexist") === undefined, "Check for no data exists" );
 
 	var data = jQuery.data(div);
 	same( data, { "test": "success" }, "Return complete data set" );
@@ -57,18 +60,25 @@ test(".data()", function() {
 })
 
 test(".data(String) and .data(String, Object)", function() {
-	expect(22);
-	var div = jQuery("#foo");
-	equals( div.data("test"), undefined, "Check for no data exists" );
+	expect(23);
+	var div = jQuery("<div/>");
+
+	ok( div.data("test") === undefined, "Check for no data exists" );
+
 	div.data("test", "success");
 	equals( div.data("test"), "success", "Check for added data" );
+
 	div.data("test", "overwritten");
 	equals( div.data("test"), "overwritten", "Check for overwritten data" );
+
 	div.data("test", undefined);
 	equals( div.data("test"), "overwritten", "Check that data wasn't removed");
+
 	div.data("test", null);
 	ok( div.data("test") === null, "Check for null data");
 
+	ok( div.data("notexist") === undefined, "Check for no data exists" );
+
 	div.data("test", "overwritten");
 	var hits = {test:0}, gets = {test:0};