How can I test for a NULL value?

I want to know how to test for a a NULL value as I discovered that null values aren't accounted for while doing some unit tests. How could i change this to check for a null value?

    public class MemberComparator implements Comparator<Member> {

    private final String clientId;

    public MemberComparator(String clientId) {
        this.clientId = clientId;
    }

    /* then = ?
        else = :
     */
    @Override
    public int compare(Member m1, Member m2) {
        if (m1.getClientId().startsWith(clientId)) {
            return m2.getClientId().startsWith(clientId) ? m1.getClientId().compareTo(m2.getClientId())
                    : -1;
        } else {
            return m2.getClientId().startsWith(clientId) ? 1
                    : m1.getClientId().compareTo(m2.getClientId());
        }
    }