Fix #14101: $().data() should be undefined, not null

- Backported 30eee925db for 1.x

src/data.js

@@ -272,8 +272,7 @@ jQuery.extend({
 
 jQuery.fn.extend({
 	data: function( key, value ) {
-		var i, name,
-			data = null,
+		var i, name, data,
 			elem = this[0],
 			attrs = elem && elem.attributes;
 
@@ -319,7 +318,7 @@ jQuery.fn.extend({
 
 			// Gets one value
 			// Try to fetch any internally stored data first
-			elem ? dataAttr( elem, key, jQuery.data( elem, key ) ) : null;
+			elem ? dataAttr( elem, key, jQuery.data( elem, key ) ) : undefined;
 	},
 
 	removeData: function( key ) {