[HttpFoundation] Fix basic authentication in url with PHP-FPM

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | dispute
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

`getUser()` and `getPassword()` from `Request` are broken when using PHP-FPM because of the lack of `$_SERVER['PHP_AUTH_USER']` and `$_SERVER['PHP_AUTH_PW']`. This PR fixes the issue.

However, now an empty password will return an empty string (which is the expected behavior of `ServerBag`) instead of `NULL`. The test is updated accordingly, but should we consider this as a breakage?

This issue was spotted by using basic auth via the Illuminate component of Laravel and is present from v2.1.0 to master.