Fix ValidationError caused by missing endCursor

[roskakori]

Steps to reproduce

Run:

pytest tests/test_done_project_items_info.py::test_can_resolve_done_project_items_info

Actual result

FAILED tests/test_done_project_items_info.py::test_can_resolve_done_project_items_info - pydantic_core._pydantic_core.ValidationError: 1 validation error for NodeByIdInfo
node.items.pageInfo.endCursor
  Input should be a valid string [type=string_type, input_value=None, input_type=NoneType]
    For further information visit https://errors.pydantic.dev/2.10/v/string_type

Expected result

Test passes.

Related issues

Revealed during #45.