ChangeStreamIterable withDocumentClass throws exception because convertResultsCallback looks for resumeToken

1. Call `watch` with a `$replaceRoot` pipeline and `withDocumentClass` and pass a class with a registered codec.
2. Observe that a resume token cannot be found in the replaced document, even when the end user does not want resume token functionality.

The core issue is in AsyncChangeStreamBatchCursor convertResultsCallback. It should not throw an exception if a resume token cannot be found (very surprising and undocumented).