GetFlagEnumValues is not safe to use

Nobody has claimed this yet.

It declares to return `IEnumerable`, so I except it is never null (https://rules.sonarsource.com/csharp/RSPEC-1168).

But `MyEnum.FromValue(0)` returns null.

```
public sealed class MyEnum : SmartFlagEnum
{
public static readonly MyEnum X= new MyEnum(1);
public static readonly MyEnum Y= new MyEnum(2);

private MyEnum (int value, [CallerMemberName] string name = null)
: base(name, value)
{
}
}
```

Start by locating GetFlagEnumValues and the FromValue(0) path described in the issue. Compare their behavior with the RSPEC-1168 non-null expectation; the issue is done when the reported null result is resolved and the method's IEnumerable contract is upheld.

Written by the indexing model from the issue text.