Avoid base64-decoding an empty user_avatar.data

Fixes issue #413, at least the non-mouse related part. I was only
able to reproduce it by trying to use the "Edit your avatar" menu item
after choose "I don't want an avatar".

This "I don't want" menu item is weird however and reappears when
the user exits and reruns the chooser, so there's probably still
some improvements needed with this menu item.

The "Upload an avatar" menu item issue is likely totally unrelated.
Please don't combine unrelated issues in the same bug report.