Resolving DjangoConnectionField when return None

* **What is the current behavior?**
DjangoConnectionField resolves all objects of the requested DjangoObjectTypeNode when we return none in the resolver!

* **Steps to reproduce:**
```
class Query(object):
test = DjangoConnectionField(SomeNode)

def resolve_test(self, info, **kwargs):
return None
```

* **What is the expected behavior?**
Should return an empty edges-nodes

* **Please tell us about your environment:**
- Version: 2.15.0

* **Notes:**
Same behavior in mutations