Labels: Handle empty sets

Fixes #15184

Author: scottgonzalez

tests/unit/core/core.js

@@ -142,7 +142,7 @@ QUnit.test( "uniqueId / removeUniqueId", function( assert ) {
 } );
 
 QUnit.test( "Labels", function( assert ) {
-assert.expect( 2 );
+assert.expect( 3 );
 
 var expected = [ "1", "2", "3", "4", "5", "6", "7", "8", "9", "10" ];
 var dom = $( "#labels-fragment" );
@@ -165,6 +165,8 @@ QUnit.test( "Labels", function( assert ) {
 // Detach the dom to test on a fragment
 dom.detach();
 testLabels( "document fragments" );
+
+assert.equal( $().labels().length, 0, "No element" );
 } );
 
 ( function() {

ui/labels.js

@@ -27,6 +27,10 @@
 return $.fn.labels = function() {
 var ancestor, selector, id, labels, ancestors;
 
+if ( !this.length ) {
+return this.pushStack( [] );
+}
+
 // Check control.labels first
 if ( this[ 0 ].labels && this[ 0 ].labels.length ) {
 return this.pushStack( this[ 0 ].labels );