Fix bug when accessing .data() on an empty set. Return null rather than throwing exception.

src/data.js

@@ -129,8 +129,8 @@ jQuery.extend({
 
 jQuery.fn.extend({
 	data: function( key, value ) {
-		if ( typeof key === "undefined" && this.length ) {
-			return jQuery.data( this[0] );
+		if ( typeof key === "undefined" ) {
+			return this.length ? jQuery.data( this[0] ) : null;
 
 		} else if ( typeof key === "object" ) {
 			return this.each(function() {

test/unit/data.js

@@ -65,7 +65,10 @@ test(".data()", function() {
 
 	var div = jQuery("#foo");
 	div.data("test", "success");
-	same( div.data(), {test: "success"}, "data() get the entire data object" )
+	same( div.data(), {test: "success"}, "data() get the entire data object" );
+
+	var nodiv = jQuery("#unfound");
+	equals( nodiv.data(), null, "data() on empty set returns null" );
 })
 
 test(".data(String) and .data(String, Object)", function() {