When you choose your profile upon selecting your account; you get 2 options, one with your actual MC profile and one where it is just blank.

When you have two accounts in the launcher, and you switch between the two, you get prompted to "Choose your profile"

You get two options, one with your actual MC user, and one that is just blank?

Once you log in and, you cannot enter any server, however, you can host your own LAN server, your username is also "--assetIndex"
Not sure if this is a bug or is intentional, but here it is.