sub post title for alt tag if alt tag is empty

/**
 * Get accessible post thumbnail images - fills alt attribute with post title if alt attribute is empty
 *
 * @param [type] $id The post ID being called.
 * @param [type] $size The size of the image to call. Default thumbnail.
 *
 * @return void
 */
function a11y_images_with_alt( $id, $size = 'thumbnail' ) {
	$alt = get_post_meta( get_post_thumbnail_id( $id ), '_wp_attachment_image_alt', true );

	if ( empty( $alt ) ) :
		$alt = get_the_title();
	endif;

	echo '<a href="' . esc_url( get_the_permalink() ) . '">' . get_the_post_thumbnail( get_the_ID(), $size, array( 'alt' => $alt ) ) . '</a>';
}