Missing nullability warnings for invalid constant values

[TessenR]

Version Used:

Branch master (14 Jan 2021)
Latest commit f3d7c35 by Julien Couvreur:
Don't check initializer on static record ctor (#50176)

Steps to Reproduce:

Compile and run the following code

#nullable enable
string s = C.MyConst;
s.ToString();

class C
{
  public const string MyConst = null;
}

Expected Behavior:
Warning for public const string MyConst = null;

Actual Behavior:
No warnings. The program crashes at runtime with a NullReferenceExceptions

[Youssef1313]

Possibly the same issue as #29995 and also #46805