Consider all mime-parts to check if message is empty

Previously only `parts` and `body` were checked, so mime-parts that were
classified into `attachments` and `inline_parts` didn't count – thus
messages that contained only those parts were shown blank.

program/actions/mail/show.php

@@ -621,7 +621,10 @@ public static function message_full_headers($attrib)
      */
     public static function message_body($attrib)
     {
-        if (empty(self::$MESSAGE) || (empty(self::$MESSAGE->parts) && empty(self::$MESSAGE->body))) {
+        // Exit early if there's no content to be shown anyway.
+        // `mime_parts` also includes a message's body, even if it originally
+        // was the only part of the message.
+        if (empty(self::$MESSAGE) || empty(self::$MESSAGE->mime_parts)) {
             return '';
         }