Offer class incorrectly assumed the merchant id always existed.

We were having a problem using this because the Amazon responses wouldn't always have a merchant id, which would cause this notice:
> Notice: Trying to get property of non-object in /path/vendor/zendframework/zendservice-amazon/library/ZendService/Amazon/Offer.php on line 78

Offer response from Amazon:

```
[0] => ZendService\Amazon\Offer Object
                        (
                            [MerchantId] => 
                            [MerchantName] => 
                            [GlancePage] => 
                            [Condition] => New
                            [OfferListingId] => imHEHzRFLdt73yrifok3lj1%2BZmJGDaw2l7Fg8itPlHmFvTaV6QYiyxZtiR7fJk39zVduT5puzBnzoEe3ag009ieGvSciYKiIy8r%2F%2BTcfqComwqtz1k44CjqjQtAo1aBhWCcmqmu9pQozm6UK3kjszXRJeLAFLLp8
                            [Price] => 5672
                            [CurrencyCode] => USD
                            [Availability] => Usually ships in 1-2 business days
                            [IsEligibleForSuperSaverShipping] => 
                        )
```