Nullability is weird
The current nullability annotations are weird and make use of the library clunky.
Exhibit a:
```csharp
Validator.Check()
.That(someValue).IsNotNull()!.IsNotEmpty()
.ElseThrow();
```
Exhibit b:
```csharp
Validator.Check()
.That(someValue)!.IsNotEmpty().When(someValue != null)
.ElseThrow();
```
I'd prefer not having to resort to the ! operator.
issue