PadDatabase.all_files() calls for all groups

See here: https://gitlab.idiap.ch/bob/bob.pad.base/blob/1475c8ea7ebec7243b4f6d6fdcaadd8d21730a43/bob/pad/base/database/database.py#L133

Some databases may not have all the groups available. Maybe it's better to pass groups=None as default.