Set a default value for array and map types

If a response has a property foo that is an optional collection then doing `$model->getFoo()` when the property is not in the response causes

```Notice: Undefined index: foo in ~/google-api-php-client/src/Google/Model.php on line 76```

Given that we know about the collection we can default to an empty array instead.

The code here is a little hairy, please make sure I haven't done anything stupid.

(My editor added a bonus whitespace fix too)