[Security] Make security.providers optional

[javiereguiluz]

symfony/symfony#26787

It's useful in cases like symfony/symfony#21998:

I use custom authentication which stands on tokens without users. Therefore, I don't use UserProvider. But configuration loader requires me to provide at least one user provider.

Please make user provider optional. When some component asks for user from user provider, it should crash, so I know there is something wrong.

[javiereguiluz]

Fixed by #9828.