[Routing][XML Loader] Add a possibility to set a default value to null
merged branch Aitboudad/ticket__7335 (PR #7635)

This PR was merged into the 2.1 branch.

Discussion
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[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>

Commits
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94a9cdc [Routing][XML Loader] Add a possibility to set a default value to null