[ticket/11404] Return empty array of avatar data if $row is empty

While creating a group in the acp, the group data ($group_row) is empty.
Due to that array_combine in phpbb_avatar_manager::clean_row() will cause
PHP Warnings. In addition to that the required indexes 'avatar',
'avatar_width', 'avatar_height', and 'avatar_type' won't be defined. This
patch will solve that issue.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-11404

PHPBB3-11404