Compiler permits creation of nullable array type

[gafter]

In #31991 (comment) it is reported that the compiler permits creation of a nullable array type:

#nullable enable
public class C {
    public void M() {
    	_ = new C?(); // error (correct)
        _ = new string[50]?; // no error (incorrect)
        _ = new string[50]?[,]; // no error (incorrect)
    }
}

[gafter]

Fixed in #32431.