If you have another meta field that is storing an image (like a Header Image upload field), it might be a good idea to set the featured image fallback to that. This is especially useful when using Yoast SEO for Facebook Open Graph tags, as it uses the featured image for the og:image property.
This code uses the meta field ea_page_header as the featured image if no featured image has been set.
/**
* Set Featured Image Fallback
* @author Bill Erickson
* @link http://www.billerickson.net/code/featured-image-fallback/
*/
function ea_featured_image_fallback( $value, $post_id, $meta_key, $single ) {
if( $single && '_thumbnail_id' == $meta_key && empty( $value ) )
$value = get_post_meta( $post_id, 'ea_page_header', true );
return $value;
}
add_filter( 'get_post_metadata', 'ea_featured_image_fallback', 10, 4 );