Submitting settings without a profile image URL breaks the avatar display

[Sija]

1. Go to https://realworld.mint-lang.com/settings
2. Click Submit without providing any value in the Image field
3. You'll be redirected to your profile page with a missing avatar (rendered as <img src="" ...>)

Also, there's no validation of the profile image URL whatsoever, i.e. form saves with any value provided (like foo).