pipe: Use empty message token '--' if no message

Previously, the subject was duplicated, which is really confusing

include/class.api.php

@@ -433,7 +433,7 @@ class ApiEmailDataParser extends EmailDataParser {
         $data['source'] = 'Email';
 
         if(!$data['message'])
-            $data['message'] = $data['subject']?$data['subject']:'-';
+            $data['message'] = '--';
 
         if(!$data['subject'])
             $data['subject'] = '[No Subject]';