Fix for nulls in server hash lists throwing errors

Let's say your data source is handling a list of hashes from an API, it looks like this:

```
[
{...},
{...},
null,
{...}
]
```

When piped through SC.Store#loadRecords, the above throws an error when it gets to the null. It's easy enough to loop through and remove the nulls in the data source, but it's also a big annoyance. A null like this can be indicative of a shortcut taken by the server, often to improve performance. I think we ought to play along automatically on the client.

Are there any knock-on effects that I haven't thought of from having loadRecords return a shorter array than was passed into it?