[HttpFoundation] [Hackday] #9942 test: Request::getContent() for null value

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #9942
| License       | MIT
| Doc PR        | 

Implementing test for issue #9942 to confirm that the bug is already fixed. It's possibile to close the issue. 

Thanks also to @baiolo, @andyroid1978 for contributions at HackDay of the SymfonyConf 
This should be a more specific assertion - like `assertSame()`, which also check the type.

Note that this test doesn't prove that #9942 is fixed, as the reporter argues that `getContent()` should return `null`.