bug symfony#25100 [SecurityBundle] providerIds is undefined error when firewall provider is not specified (karser)

This PR was squashed before being merged into the 3.4 branch (closes symfony#25100).

Discussion
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[SecurityBundle] providerIds is undefined error when firewall provider is not specified

| Q             | A
| ------------- | ---
| Branch?       |  3.4
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

If I don't specify user provider on a firewall, it throws an error: `Undefined variable: providerIds`. That's because of $providerIds in [3.4-RC1](https://github.com/symfony/symfony/blob/v3.4.0-RC1/src/Symfony/Bundle/SecurityBundle/DependencyInjection/SecurityExtension.php#L696) that is not defined.
```
    private function createSwitchUserListener($container, $id, $config, $defaultProvider, $stateless)
    {
        $userProvider = isset($config['provider']) ? $this->getUserProviderId($config['provider']) : ($defaultProvider ?: $this->getFirstProvider($id, 'switch_user', $providerIds));
```
In fact, in [master](https://github.com/symfony/symfony/blob/master/src/Symfony/Bundle/SecurityBundle/DependencyInjection/SecurityExtension.php#L599) it's fixed

If I add a provider to the firewall, this error doesn't reproduce:
```
    secured_area:
        switch_user: true
        provider: chain_provider
```

Commits
-------

aedf897 [SecurityBundle] providerIds is undefined error when firewall provider is not specified

Author: chalasr