Fixed QuerySet.raw_aggregate() when some documents don't contain all model fields

[Jibola]

If there exists a document that fits the query criteria but does not have most of the expected keys, it will force every subsequent document to also not render those values.

class Table(models.Model):
    brand = models.CharField(...)
    volume = models.IntegerField(...)
    # intentionally left out unit

Documents:

{
  _id: 1,
  "brand": "SPAM"
},
{
  _id: 2,
  "brand": "SPAM",
  "volume": 16,
  "unit": "oz",
  ...
}

In the above example, Table.objects.raw_aggregate([]) would yield...

SPAM_OBJECT_1.brand == "SPAM" 
SPAM_OBJECT_2.brand == "SPAM"
SPAM_OBJECT_2.volume == 16 <-- Additional DB query
SPAM_OBJECT_2.unit (AttributeError)