Fixed the NullLogger to implement the HttpKernel interface again

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no (reverting one introduced by mistake)
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | n/a

This BC breaking mistake has been reported in https://github.com/schmittjoh/JMSSecurityExtraBundle/pull/105#issuecomment-12908659