Hi,
That's normal.
When the text version is empty, HikaShop will generate it automatically from the HTML version if necessary (only when the "Multiple part" setting is activated in the HikaShop configuration).
Note that text version of emails is not used much recently. Almost all the email clients support HTML and thus the text version is only generated as a failsafe in the really really rare occasion that a user would get the email on an email client which doesn't supports HTML.
So I would recommend not bothering with this.