public static <K, V> Map<K, V> nullToEmpty(Map<K, V> collection) {
    return collection == null ? Collections.emptyMap() : collection;
  }
Traverses up the hierarchy to find the "root" or "parent" container.
<p/>
The root AbstractKerberosDescriptor is the first descriptor in the hierarchy with a null parent.

@return the AbstractKerberosDescriptor implementation that is found to be the root of the hierarchy.
ambari-server/src/main/java/org/apache/ambari/server/state/kerberos/AbstractKerberosDescriptor.java