fixed error in filter email when headers are not set or not an array

plugins/custer/filter_mail.php

@@ -125,8 +125,16 @@ function find_user_id_from_headers(&$args) {
 	if (!isset($args['headers'])) {
 		return null;
 	}
-	$user_id = null;
 	$headers = $args['headers'];
+	if (empty($headers)) {
+		return null;
+	}
+	if (!is_array($headers)) {
+		return null;
+	}
+
+	$user_id = null;
+
 	// isset returns false if the key exists but its value is null : 'user_id'=>null : https://stackoverflow.com/questions/3803282/check-if-value-isset-and-null
 	if (array_key_exists('user_id', $headers)) {
 		$user_id = $headers['user_id'];