This change improves the usability around Fields

See 

https://github.com/elasticsearch/elasticsearch-net/blob/b8c03eb0d714258dab5fc207a75f797a0c164a4b/src/Tests/Nest.Tests.Integration/Search/FieldTests/FieldsTest.cs#L38

For updated usages and `.Documents` now returns an empty enumerable when `.Fields()` are used on the `SearchDescriptor`