Autocomplete: Make sure we have a response before trying to check the…

… length. Fixes #6147 - Autocomplete: Errors on empty responses.

ui/jquery.ui.autocomplete.js

@@ -268,7 +268,7 @@ $.widget( "ui.autocomplete", {
 	},
 
 	_response: function( content ) {
-		if ( content.length ) {
+		if ( content && content.length ) {
 			content = this._normalize( content );
 			this._suggest( content );
 			this._trigger( "open" );