Handle GenericForeignKey class typeinfo lookup failure. (#597)

This addresses an obscure crash we're getting when defining a GenericForeignKey
subclass on a model.

Not sure how this slipped through type checking since
`helpers.lookup_class_typeinfo -> Optional[TypeInfo]` while
`.get_private_descriptor_type(type_info: TypeInfo)` so this should be a clear
type violation.

mypy_django_plugin/django/context.py

@@ -198,7 +198,12 @@ class DjangoContext:
                 # it's generic, so cannot set specific model
                 field_name = field.name
                 gfk_info = helpers.lookup_class_typeinfo(api, field.__class__)
-                gfk_set_type = helpers.get_private_descriptor_type(gfk_info, "_pyi_private_set_type", is_nullable=True)
+                if gfk_info is None:
+                    gfk_set_type = AnyType(TypeOfAny.unannotated)
+                else:
+                    gfk_set_type = helpers.get_private_descriptor_type(
+                        gfk_info, "_pyi_private_set_type", is_nullable=True
+                    )
                 expected_types[field_name] = gfk_set_type
 
         return expected_types