@Override public Expected<T> ifAbsent(final Runnable runnable){
  if (isAbsent()) {
    runnable.run();
  }
  return this;
}

@ Override public Expected < T > ifAbsent ( final Runnable runnable ) { if ( isAbsent ( ) ) { runnable . run ( ) ; } return this ; }
if a value is not present ( null ) , invoke the runnable .
