Ensure that DocumentSchema.getSubFields() returns a non null value

On August 22 2011, this line was introduced. We can assume that the intended behaviour would be to return an empty list if subFields is null, but instead, we are (still today) returning null.
Although this change is trivial, it will likely require some adjustments in the tests, as it is possible that those are expecting to get null for some cases. There's also a possibility for us to clean up the code base to avoid checking for null after getting the subfields, as it will no longer be a possibility if this is fixed.