Null pointers in parseDatum method

Hi, 

I am getting a null pointer exception in the above method as you are expecting optional members. 
This is particularly happening to me for "links". 
```
 // optional top-level member: links
        // a links object containing links related to the resource.
        JsonObject links = datum.get("links").asObject();
```

`datum.get("links")` returns null. 



