ReadFromJsonAsync with nullable value types enabled makes return type look strange

[Eneuman]

Using ReadFromJsonAsync in a nullable value type enabled .Net 5 ASP project has som strange behaviours.

For example: the following code return a MyClass? nullable object:
var problemDetails = await response.Content.ReadFromJsonAsync<MyClass>();

This code returns a int (not nullable) (what happens if it is not a a numeric value in the response):
var result = await response.Content.ReadFromJsonAsync<int>();

But this code returns a nullable string:
var result = await response.Content.ReadFromJsonAsync<string>();
Shouldn't it return a none nullable string and throw if it is null?

[Dotnet-GitSync-Bot]

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

[layomia]

Triage: consider as part of #1256.