JSON error in logs when accessing stats page for a package with no downloads.

[claycompton]

To repro:

1. Upload a brand-new package.
2. Immediate switch to that package's stats page.
3. Check the Error Logs

Result: There's a NullReferenceException logged each time the stats page is requested for a package with zero downloads. An example full stack trace can be found at https://qa.nugettest.org/Admin/Errors.axd/detail?id=6275.
Expected: The user never actually sees an "Oh, no" page, and the case seems common enough that we shouldn't actually need to log this error. Although it's a corner case, perhaps we should suppress this error to remove spurious messages from our error logs.