nobody.png is returned instead of MysteryMan

Now it is the other way around ! 😆

When requesting a non existing user's avatar with a default option of 'mm' or 'mp' the nobody.png picture is returned instead of 'mm.png'.

Example:

curl -L https://avatars.linux-kernel.at/avatar/4751ed9aae86881d2b45dd0512c3e14a?d=mm&f=y

This regression doesn't affect values of 'identicon' or 'retro'.