orm: Don't null out value on assignment

This fixes an issue where the PK of ThreadEntry will be NULLed in the
following code

```php
$entry = ThreadEntry::create(array(...));
$entry->save();
$entry->email_info = new ThreadEntryEmailInfo(array(
    'mid' => 'xyzAbc',
));
```

In the above code, the $entry->__set('email_info', <ThreadEntryEmailInfo>)
would be invoked. The ThreadEntryEmailInfo object is new, and so will cause
the local part of the relationship (`id` in this case) to
become null further down in the `__set` method.

This issue is fixed in this commit by removing the NULL assignment to the
new object. This was added in 11322766,
however, it is unclear why the null assignment is performed.