Newly added embedded documents list field is None instead of list.

Dear maintainers:
With this issue I present an unexpected scenario that I've encountered recently for discussion.

Related models for this issue:

class Drink(EmbeddedDocument):
    cost = IntField()

class Shop(Document):
    name = StringField()
    drinks = EmbeddedDocumentListField(Drink)  # Our focus in this issue

Scenario:

1. We start without the drink field on the Shop model.
   Inside the database, documents at this time does not have drink key inside.
2. Add the drinks field onto the Shop document, the model we have now is exactly what's above.
3. At this point, we execute the following query:

shop_obj = Shop.objects.only('drinks').first()

4. Here comes the bug and the reason for this issue, what should be shop_obj.drinks here ?
   I expected it to be [], but apparently it's `None.

So what should it be ? An empty list or None ? I'm not sure what's the intended outcome here.
Me personally think it should be an empty list due to the fact that it is what I've defined on the model, and if that's the case I can open a PR regarding this issue. Just need confirmation on this behavior/direction with you guys.
Thanks.

Start by reproducing the issue with the shown Drink and Shop models and the Shop.objects.only('drinks').first() query. Determine the intended value for an omitted EmbeddedDocumentListField, then add a regression test covering newly added fields and verify the resulting value matches the project decision.