code: public static void isNull (final Object aValue, @Nonnull final Supplier <? extends String> aName)
    {
      if (isEnabled ())
        if (aValue != null)
          throw new IllegalArgumentException ("The value of '" + aName.get () + "' must be null but is " + aValue);
    }
docs: Check that the passed value is <code>null</code>.
  
  @param aValue
  The value to check.
  @param aName
  The name of the value (e.g. the parameter name)
  @throws IllegalArgumentException
  if the passed value is not <code>null</code>.