Mark StringContent mediaType parameters as nullable

[MihaZupan]

Closes #83423

This PR doesn't change the behavior of MediaTypeHeaderValue? mediaType overloads where null currently means "no media type".
If we decide that's something we want to do, we can do so as a follow-up.

We have existing tests covering passing null to these consturctors.

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[wfurt]

lgtm

[campersau]

The ! could now be removed from here right?

runtime/src/libraries/System.Net.Http/tests/FunctionalTests/StringContentTest.cs

Line 134 in d2cada8

var content = new StringContent(sourceString, defaultStringEncoding, ((Headers.MediaTypeHeaderValue)null)!);