 current list of values for the supplied key. If the supplied array of new values is empty,
 method returns immediately. Method throws a <code>NullPointerException</code> if the supplied array of values is
 <code>null</code>.
 </p>
 NOTE: This implementation ignores <code>null</code> values; Any of the supplied values of <code>null</code> is ignored and not
 added to the value list. Overriding implementations may modify this behavior by redefining the
 <a href="#addNull(java.util.List)"><code>addNull(java.u