Robohash throws exception if missing background configuration #37
Description
Creating new Robohash plugin instance without background produces exception.
Drupal\avatars\Exception\AvatarException: Invalid background in Drupal\avatars_robohash\Robohash->setBackground() (line 79 of modules/contrib/avatars/avatars_robohash/src/Robohash.php).
Plugin should use a default background if no configuration is set.