Revert PR 412 and remove support for null values.

The premise of this library is that it can extend the built-in enum types in .NET, but still, it will mimic the behavior of the enum types. SmartEnums even though defined as classes, they're extended to offer value-type behavior.

This [PR](https://github.com/ardalis/SmartEnum/pull/412) enables creating a SmartEnum with a null value. This breaks the assumed contract and leads to various inconsistencies. For instance, we can no longer guarantee that SmartEnum can be used as dictionary keys.

```csharp
var types = new Dictionary();

types.Add(CustomerType.One, new()); // Will throw

public class CustomerType : SmartEnum
{
public static CustomerType One = new CustomerType("One", null);
public static CustomerType Two = new CustomerType("Two", "two");
protected CustomerType(string name, string value) : base(name, value) { }
}
```

Start by reviewing PR 412 and the SmartEnum value-handling entry points it changed. Restore the non-null value contract described in the issue and verify that a SmartEnum with a null value cannot be created and can safely be used as a dictionary key.

Written by the indexing model from the issue text.