Update get_auto_imports() to also allow returning None (#2956)

Returning `None` is the documented way to permanently disable automatic imports.

https://docs.djangoproject.com/en/5.2/howto/custom-shell/#customizing-shell-auto-imports

There is also specific handling of `None` within Django's `shell` management command.

https://github.com/django/django/blob/5.2.9/django/core/management/commands/shell.py#L170-L172

Author: jamesbeith

django-stubs/core/management/commands/shell.pyi

@@ -9,5 +9,5 @@ class Command(BaseCommand):
     def ipython(self, options: Any) -> None: ...
     def bpython(self, options: Any) -> None: ...
     def python(self, options: Any) -> None: ...
-    def get_auto_imports(self) -> list[str]: ...
+    def get_auto_imports(self) -> list[str] | None: ...
     def get_namespace(self, **options: Any) -> dict[str, Any]: ...