Fix IndexError for custom queryset managers

Sometimes Mypy crashes on this line with an IndexError.

This change builds on the change in
typeddjango#1786 by making further
use of the safer variable `typed_var`.

`typed_var` will equal `manager_instance.args` if
`manager_instance.args` is populated, but if it isn't it will have
another sensible value we can use instead.

Because that value should be populated, this prevents the crash.

mypy_django_plugin/transformers/managers.py

@@ -135,7 +135,7 @@ def _process_dynamic_method(
         ):
             ret_type = _replace_type_var(ret_type, base_that_has_method.defn.type_vars[0].fullname, typed_var[0])
             args_types = [
-                _replace_type_var(arg_type, base_that_has_method.defn.type_vars[0].fullname, manager_instance.args[0])
+                _replace_type_var(arg_type, base_that_has_method.defn.type_vars[0].fullname, typed_var[0])
                 for arg_type in args_types
             ]
     if base_that_has_method.self_type: