Accessibility: Customize: Add an empty `alt` attribute to the `<img>` tag used as a hidden placeholder in the Customizer preview when no logo is chosen.

Per accessibility best practices, it is recommended that all HTML `<img>` elements have an `alt` attribute. Any decorative images should have an empty `alt` attribute (`alt=""`).

Props laxman-prajapati, sabernhardt, audrasjb.
Fixes #51846.
Built from https://develop.svn.wordpress.org/trunk@49949


git-svn-id: http://core.svn.wordpress.org/trunk@49648 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/general-template.php

@@ -1081,7 +1081,7 @@ function get_custom_logo( $blog_id = 0 ) {
 	} elseif ( is_customize_preview() ) {
 		// If no logo is set but we're in the Customizer, leave a placeholder (needed for the live preview).
 		$html = sprintf(
-			'<a href="%1$s" class="custom-logo-link" style="display:none;"><img class="custom-logo"/></a>',
+			'<a href="%1$s" class="custom-logo-link" style="display:none;"><img class="custom-logo" alt="" /></a>',
 			esc_url( home_url( '/' ) )
 		);
 	}

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.7-alpha-49948';
+$wp_version = '5.7-alpha-49949';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.