public String getDomainRoot(final String domain) {
        return getDomainRoot(domain, null);
    }
Returns registrable part of the domain for the given domain name or {@code null}
if given domain represents a public suffix.

@param domain
@return domain root
ngrinder-runtime/src/main/java/org/apache/hc/client5/http/psl/PublicSuffixMatcher.java