[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.

PHPBB3-11404
[ticket/11404] Use a default data row if $row is empty in clean_row()

A statically defined $default_row will be used inside the
phpbb_avatar_manager::clean_row() method if the $row passed to it is
empty.

PHPBB3-11404
[ticket/11404] Convert manager_test to UNIX line endings

PHPBB3-11404
[ticket/11404] Add tests for phpbb_avatar_manager::clean_row()

PHPBB3-11404
[ticket/11404] Remove version ID from manager_test.php file header

PHPBB3-11404
Merge remote-tracking branch 'remotes/Marc/ticket/11404' into develop

# By Marc Alexander
# Via Marc Alexander
* remotes/Marc/ticket/11404:
  [ticket/11404] Remove version ID from manager_test.php file header
  [ticket/11404] Add tests for phpbb_avatar_manager::clean_row()
  [ticket/11404] Convert manager_test to UNIX line endings
  [ticket/11404] Use a default data row if $row is empty in clean_row()
  [ticket/11404] Return empty array of avatar data if $row is empty