My suggestion is to modify your main view to list only Product nodes with a taxonomy term selected, then use an attachment to your main view that lists Product nodes with no taxonomy term. The attachment would be displayed after your main view.
1. Filter your main view to list only Product nodes with a taxonomy term value
In your main view's configuration, under Filter Criteria, add a filter for your taxonomy term field (choose either dropdown or autocomplete), and select "Is not empty (NOT NULL)".
2. Create the attachment that lists Product nodes with no taxonomy term value
Add an Attachment display to your view.
Under Attachment settings, set Attach to to your main view display (in this example the main view is Page), and set the Attachment position to After.
In the attachment display's Filter Criteria, change the taxonomy term filter to "Is empty (NULL)". Make sure to override this setting for the attachment display only or it will apply to the main view as well.
Note about the attachment header
Your attachment view will be displayed right after your main view, so you may want to add a Header to the attachment (make sure to override to the attachment display only).
In this example I used a Text area.
Alternatively, you could add a label to the taxonomy term field. Make sure to override this setting for the attachment display only or it will apply back to the main view.
Your main view should now list your Product nodes by taxonomy term in ascending order, and your Product nodes with no value selected for the taxonomy at the bottom of the list.