JsonRenderer should put a 'null' instead of just 2 commas together

```
JsonRenderer doesn't generate valid JSON as it will create something like

,,

where it should be

,null,

Attached is a diff that will resolve the issue
```

Original issue reported on code.google.com by `ba...@guild1.com` on 27 Jun 2012 at 10:29

Attachments:
- [diff.txt](https://storage.googleapis.com/google-code-attachments/google-visualization-java/issue-39/comment-0/diff.txt)
