code: @Pure
  	@SuppressWarnings("unchecked")
  	protected static <VALUET> VALUET maskNull(VALUET value) {
  		return (value == null) ? (VALUET) NULL_VALUE : value;
  	}
docs: Mask the null values given by the used of this map.
  
  <p>This method replaces the <code>null</code> value by
  the internal representation {@link #NULL_VALUE}.
  
  @param <VALUET> is the type of the value.
  @param value is the value given by the user of this map.
  @return the internal representation of the value.
  @see #unmaskNull(Object)