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 );