Only search for a avatar if loginName isn't empty

70c745d6147e465ce1e34b6037eb7dedb8e77f27 introduced fetching user
avatars via AccountsService. It's seems pointless to search for an
avatar with an empty user name, e.g. an invalid user, though.
Furthermore it makes kusertest fail with the following message if
ACCOUNTS_SERVICE_ICON_DIR exists:
'Actual (invalidKUser.faceIconPath()): "/var/lib/AccountsService/icons/"
Expected (QString())'

REVIEW: 129351