Adds a check for any registered post_meta keys

Once the query has been run to fetch any post_meta keys from the table, this checks if that array contains values.
If it does not, it fetchs any registered post meta keys, and uses that array instead.

src/wp-admin/includes/template.php

@@ -731,6 +731,11 @@ function meta_form( $post = null ) {
 		);
 	}
 
+	if ( !$keys ){
+		$registered_meta = get_registered_meta_keys( 'post', $post->post_type );
+		$keys = array_keys( $registered_meta );
+	}
+
 	if ( $keys ) {
 		natcasesort( $keys );
 	}