Changed getAllOptions() to allow "0"

Let's say we want to parse an option with value "0":

```php
    $parser->addRule('port::');
    $args = array('progname', '--port=0');
    $parser->parse($args);

    //when we try to get option, it returns correct value
    var_dump($parser->getOption('port')); //$args

    //when we try to get alloptions,
    //an option with value '0' is not included
    var_dump($parser->getAllOptions()); //array(0) {}
```

I think it is not correct behavior.

This pull-request fixes it.