Are these fine if they're null? Should they not be `undefined`?

Maybe better to do:
```js
if (self.resolveExplicitHashKey)
  data.ExplicitHashKey = self.resolveExplicitHashKey(chunk, encoding)
if (self.resolveSequenceNumberForOrdering)
  data.SequenceNumberForOrdering = self.resolveSequenceNumberForOrdering()
```