[Validator] Made sure that context changes don't leak out of (Contextual)ValidatorInterface

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11046
| License       | MIT
| Doc PR        | -
@Tobion After thinking about this some more, I figured that you are right. Passing `null` to the method of an interface that technically does not accept `null` values is bogus, and introduces a dependency to the implementation. I changed the interface now.