Ive had paper amended returns kicked back as "incomplete" when no numbers were in any columns, not sure if the same thing happens with Efiled amended returns.

Cant you send the 8606 in alone? feels like I remember that being a thing.

As Lisa said the 8606 can be sent without a tax return, there is a place for signature and dates at the bottom of the form.

You can send the 8606 alone if you're not required to file a tax return.

I find a 1040x with no numbers weird, so, the numbers go in.  Thanks.

You send in the 8606 alone if you're not required to file a tax return.  This taxpayer IS required to (and did) file a tax return but left out the 8606.  I'd love to file it alone but I think that's asking for trouble...

You did send in a tax return so now just send in the 8606.

That is your answer,  you did file a tax return  so now send the 8606 on its own.