[ParamFetcher] Allow null attributes to be overridden

I want to be able to use the ParamConverters on values added by the QueryParam annotation, but they're optional. When optional, they're in the attributes bag as null.

This allows a configuration such as:

```php
    /**
     * @Rest\QueryParam(name="customer", nullable=true, description="Customer (optional)")
     * @Feb\ParamConverter("customer")
     * @Rest\View
     *
     * @param Request $request
     * @param Customer|null $customer
     * @return CreateView
     */
    public function createAction(Request $request, Customer $customer = null)
    {
```

It may be more appropriate to check additionally if nullable=true is set, though this means adding an additional service dependency on the ParamFetcherListener.