Display a default image for custom-logo

When you display the custom logo you can check if is set, if not you display another image. You can achieve this with the following code featured in the documentation. I adapted it to suit your needs:

$custom_logo_id = get_theme_mod( 'custom_logo' );
$logo = wp_get_attachment_image_src( $custom_logo_id , 'full' );
if ( has_custom_logo() ) {
        echo '<img src="'. esc_url( $logo[0] ) .'">';
} else {
        echo '<img src="'. get_stylesheet_directory_uri() . '/images/header.png' .'">';
}