Make sure .data() can be called on an empty set. Fixes #6434.

Author: rdworth

src/data.js

@@ -101,9 +101,12 @@ jQuery.extend({
 
 jQuery.fn.extend({
 	data: function( key, value ) {
-		if ( typeof key === "undefined" && this.length ) {
-			return jQuery.data( this[0] );
-
+		if ( typeof key === "undefined" ) {
+			if ( this.length ) {
+				return jQuery.data( this[0] );
+			} else {
+				return undefined;
+			}
 		} else if ( typeof key === "object" ) {
 			return this.each(function() {
 				jQuery.data( this, key );

test/unit/data.js

@@ -60,17 +60,22 @@ test("jQuery.data", function() {
 });
 
 test(".data()", function() {
-	expect(1);
+	expect(2);
+
+	ok( jQuery().data() === undefined, "Check that .data() called on an empty set returns undefined" );
 
 	var div = jQuery("#foo");
 	div.data("test", "success");
 	same( div.data(), {test: "success"}, "data() get the entire data object" )
 })
 
 test(".data(String) and .data(String, Object)", function() {
-	expect(23);
-	var div = jQuery("<div/>");
+	expect(25);
+
+	ok( jQuery().data("foo") === undefined, "Check that .data(String) called on an empty set returns undefined" );
+	ok( jQuery().data("foo", {"foo":"bar"}).data("foo") === undefined, "Check that .data(String, Object) called on an empty set is chainable" );
 
+	var div = jQuery("<div/>");
 	ok( div.data("test") === undefined, "Check for no data exists" );
 
 	div.data("test", "success");