[fix] Set default value for YNH_APP_BACKUP_DIR in ynh_backup helper
[fix] Set default value for YNH_APP_BACKUP_DIR in ynh_backup helper

YunoHost/moulinette-yunohost,YunoHost/moulinette-yunohost,YunoHost/yunohost,YunoHost/moulinette-yunohost,YunoHost/yunohost,YunoHost/yunohost,YunoHost/yunohost,YunoHost/moulinette-yunohost,YunoHost/moulinette-yunohost