Fix header layout if option is not set (#578)

Author: albig

header.php

@@ -36,7 +36,7 @@
 		<a class="skip-link screen-reader-text" href="#primary"><?php esc_html_e( 'Skip to content', 'sunflower' ); ?></a>
 
 		<?php
-		$sunflower_header_layout = sunflower_get_setting( 'sunflower_header_layout' ) ?? 'standard';
+		$sunflower_header_layout = sunflower_get_setting( 'sunflower_header_layout' ) ? sunflower_get_setting( 'sunflower_header_layout' ) : 'standard';
 
 		get_template_part( 'template-parts/header', $sunflower_header_layout );
 		?>