fix #1980 GetAsync fails when specifying a null index.

```csharp
var response = await elasticClient.GetAsync<T>(id, s =>
  s.Index(null));
```

Would not default index back to inferred type of T.

This PR makes specifying null for required route values a noop

For optional routevalues null still clears the routevalue so the
following still works

```csharp
c => c.Index(project, i => i.Id(null))
```

will reset the id on and send the index request to
`/inferredindex/inferredtype`

Author: Mpdreamz