No nullable warning when combining static generic fields and nullable generic type instantiations

**Version Used**: Main from 11 Jun 2024

**Steps to Reproduce**:

The basic idea comes from the design of nullable reference types being erased and unknown to the runtime.
With a static storage in a generic type, the data for `C` and `C` is shared. However, this is neither understood nor warned about.

```
using System;
using System.Collections.Generic;

C.Instance[0] = "hi";
C.Instance[0] = null;
Console.Write(C.Instance[0].Length);

public class C {
public static Dictionary Instance {get;set;}
static C() {
Instance = new Dictionary();
}
}
```

https://sharplab.io/#v2:D4AQTAjAsAUCAMACEEAsBuWsDCAeAygC4BOAlgHYDmAfAHQCS5AzoQIbkDGApgNrwC6iALyIARAAtSozDDxEyVAPx1GLdtz6CR5AK4AbPTOwB7Zsb1daAdTKEuACjkkKNBszadeA2gBkuVQnEAShlYEABmZDBEPAAVakQAb1hEVORIlAA2RAARUg5CUlNWYgBPXApCABp4xFUPbiTKLkJ0Jhb0AF8UtKyY+yCknrS0+vUuYURyLgB3XPzC4rKK8mrEeIGZEe6YHawYIA

**Expected Behavior**:
**Any** kind of warning on using static data in generics with a nullable type instantiation.

**Actual Behavior**:
System.NullReferenceException: Object reference not set to an instance of an object.

Start by compiling and running the provided C# reproduction, then investigate Roslyn's nullable analysis for static fields in generic types and nullable type instantiations. Done means the compiler reports some warning for the demonstrated unsafe use, with the expected behavior validated against the sample.

Written by the indexing model from the issue text.