public static ApplicationRoles fromString(String hostRole, String containerRole) {
        if(Strings.isNullOrEmpty(hostRole) || Strings.isNullOrEmpty(containerRole)) {
            return null;
        }
        return new ApplicationRoles(hostRole, containerRole);
    }
@return an ApplicationRoles instance if both hostRole and containerRole is non-empty, <code>null</code> otherwise
config-model-api/src/main/java/com/yahoo/config/model/api/ApplicationRoles.java