Nullability of static field doesn't seem to be tracked

[jcouv]

Example

public class C 
{
    static readonly string? s = null;
    public void M() 
    {
        if (s != null)
        {
         	s.ToString(); // warning CS8602: Possible dereference of a null reference.
        }
    }
}

Found in nullable dogfood.
Tagging @cston

[jcouv]

Here's another symptom, likely from the same cause:

public class C 
{
    static readonly string[] x = new string[] { null }; // no warning
}

[cston]

The warning is not reported in the second example because we're currently skipping NullableWalker for implicit members. If the initializer is moved to an explicit .cctor, the warning is reported. We should have a separate bug to handle implicit members such as the default .ctor and .cctor.

[cston]

Logged #31139 for the issue with implicit constructors.