Customize: Prevent absent site icon link from outputting an empty string as href in customizer; use /favicon.ico as fallback.

An empty string causes some browsers to use the current URL as the `href`. When using `history.replaceState()` Chrome will re-fetch the favicon with each call, meaning that `customize.php` gets hit with wasted requests which tax the server.

Fixes #38377.

Built from https://develop.svn.wordpress.org/trunk@38901


git-svn-id: http://core.svn.wordpress.org/trunk@38844 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/js/customize-controls.js

@@ -3333,7 +3333,7 @@
 		 * @param {object} attachment
 		 */
 		setImageFromAttachment: function( attachment ) {
-			var sizes = [ 'site_icon-32', 'thumbnail', 'full' ],
+			var sizes = [ 'site_icon-32', 'thumbnail', 'full' ], link,
 				icon;
 
 			_.each( sizes, function( size ) {
@@ -3347,8 +3347,13 @@
 			// Set the Customizer setting; the callback takes care of rendering.
 			this.setting( attachment.id );
 
+			if ( ! icon ) {
+				return;
+			}
+
 			// Update the icon in-browser.
-			$( 'link[sizes="32x32"]' ).attr( 'href', icon.url );
+			link = $( 'link[rel="icon"][sizes="32x32"]' );
+			link.attr( 'href', icon.url );
 		},
 
 		/**
@@ -3365,7 +3370,7 @@
 			this.params.attachment = {};
 			this.setting( '' );
 			this.renderContent(); // Not bound to setting change when emptying.
-			$( 'link[rel="icon"]' ).attr( 'href', '' );
+			$( 'link[rel="icon"][sizes="32x32"]' ).attr( 'href', '/favicon.ico' ); // Set to default.
 		}
 	});