[Routing][XML Loader] Add a possibility to set a default value to null

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/issues/7335
| License       | MIT

Example:
  
    <route id="acme_user_show" pattern="/{id}">
        <default key="_controller">AcmeUserBundle:User:show</default>
        <default key="id" xsi:nil="true" />
    </route>