feat: MPTT relations compatibility

[Eraldo]

Description

Trying to get an author and then the total count of the children of that author breaks when there are no children present but requested in the query. Error:

"message": "An unknown error occurred."

The PR includes a test test_nexted_children.py, which illustrates the use case and resulting error.

FYI: Disabling the DjangoOptimizerExtension makes the test pass without any issue.

Types of Changes

• [x] Bug reproduction for 3rd party compatibility

Reproduction

I crated a PR with a issue reproduction: https://github.com/strawberry-graphql/strawberry-django/pull/551

Context

Error stack trace

FAILED    [100%]ERROR 

GraphQL request:10:25
 9 |                     children {
10 |                         totalCount
   |                         ^
11 |                         edges {
Traceback (most recent call last):
  File "/Users/eraldo/Library/Caches/pypoetry/virtualenvs/strawberry-graphql-django-4L3LqJ2E-py3.12/lib/python3.12/site-packages/graphql/execution/execute.py", line 521, in execute_field
    result = resolve_fn(source, info, **args)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/eraldo/Inbox/temp/sandbox/strawberry-django/strawberry_django/optimizer.py", line 1075, in resolve
    ret = _next(root, info, *args, **kwargs)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/eraldo/Library/Caches/pypoetry/virtualenvs/strawberry-graphql-django-4L3LqJ2E-py3.12/lib/python3.12/site-packages/strawberry/schema/schema_converter.py", line 740, in _resolver
    return _get_result_with_extensions(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/eraldo/Library/Caches/pypoetry/virtualenvs/strawberry-graphql-django-4L3LqJ2E-py3.12/lib/python3.12/site-packages/strawberry/schema/schema_converter.py", line 727, in extension_resolver
    return reduce(
           ^^^^^^^
  File "/Users/eraldo/Library/Caches/pypoetry/virtualenvs/strawberry-graphql-django-4L3LqJ2E-py3.12/lib/python3.12/site-packages/strawberry/schema/schema_converter.py", line 722, in wrapped_get_result
    return _get_result(
           ^^^^^^^^^^^^
  File "/Users/eraldo/Library/Caches/pypoetry/virtualenvs/strawberry-graphql-django-4L3LqJ2E-py3.12/lib/python3.12/site-packages/strawberry/schema/schema_converter.py", line 678, in _get_result
    return field.get_result(
           ^^^^^^^^^^^^^^^^^
  File "/Users/eraldo/Library/Caches/pypoetry/virtualenvs/strawberry-graphql-django-4L3LqJ2E-py3.12/lib/python3.12/site-packages/strawberry/field.py", line 227, in get_result
    return self.base_resolver(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/eraldo/Library/Caches/pypoetry/virtualenvs/strawberry-graphql-django-4L3LqJ2E-py3.12/lib/python3.12/site-packages/strawberry/types/fields/resolver.py", line 203, in __call__
    return self.wrapped_func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/eraldo/Inbox/temp/sandbox/strawberry-django/strawberry_django/resolvers.py", line 116, in inner_wrapper
    return f(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^
  File "/Users/eraldo/Inbox/temp/sandbox/strawberry-django/strawberry_django/resolvers.py", line 83, in sync_resolver
    retval = resolver(*args, **kwargs)
             ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/eraldo/Inbox/temp/sandbox/strawberry-django/strawberry_django/relay.py", line 55, in total_count
    assert self.nodes is not None
AssertionError

tests/relay/lazy/test_nested_children.py:20 (test_nested_children_total_count)
def test_nested_children_total_count():
        parent = RelayAuthor.objects.create(name="Parent")
        RelayAuthor.objects.create(name="Child", parent=parent)
        query = """
        query {
            authors(last: 1) {
                totalCount
                edges {
                    node {
                        id
                        name
                        children {
                            totalCount
                            edges {
                                node {
                                    id
                                    name
                                }
                            }
                        }
                    }
                }
            }
        }
        """

        result = schema.execute_sync(query)
>       assert not result.errors
E       AssertionError: assert not [GraphQLError('', locations=[SourceLocation(line=10, column=25)], path=['authors', 'edges', 0, 'node', 'children', 'totalCount'])]
E        +  where [GraphQLError('', locations=[SourceLocation(line=10, column=25)], path=['authors', 'edges', 0, 'node', 'children', 'totalCount'])] = ExecutionResult(data={'authors': {'totalCount': 2, 'edges': [{'node': {'id': 'QXV0aG9yVHlwZToy', 'name': 'Child', 'children': {'totalCount': None, 'edges': []}}}]}}, errors=[GraphQLError('', locations=[SourceLocation(line=10, column=25)], path=['authors', 'edges', 0, 'node', 'children', 'totalCount'])], extensions={}).errors

lazy/test_nested_children.py:48: AssertionError

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