Duplication on null conversion

We repeat ourselves, and classify an expression such as `object o = null` in two different ways:

[10.2.7](https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/conversions.md#1027-null-literal-conversions) Null literal conversions
> An implicit conversion exists from the null literal to any reference type or nullable value type.

[10.2.8](https://github.com/dotnet/csharpstandard/blob/draft-v8/standard/conversions.md#1028-implicit-reference-conversions) Implicit reference conversions includes:

- From the null literal (§6.4.5.7) to any reference-type.

We should classify it as one or the other.