Fix namedtuple docstring/signature issues, fixes #1506

jedi/inference/gradual/typeshed.py

@@ -129,6 +129,9 @@ def import_module_decorator(func):
 
 
 def _try_to_load_stub_cached(inference_state, import_names, *args, **kwargs):
+    if import_names is None:
+        return None
+
     try:
         return inference_state.stub_module_cache[import_names]
     except KeyError:

jedi/plugins/stdlib.py

@@ -44,8 +44,6 @@ from operator import itemgetter as _itemgetter
 from collections import OrderedDict
 
 class {typename}(tuple):
-    '{typename}({arg_list})'
-
     __slots__ = ()
 
     _fields = {field_names!r}

test/test_inference/test_stdlib.py

@@ -77,6 +77,7 @@ def test_namedtuple_infer(Script):
 
     assert d1.get_line_code() == "class Foo(tuple):\n"
     assert d1.module_path is None
+    assert d1.docstring() == 'Foo(id, timestamp, gps_timestamp, attributes)'
 
 
 def test_re_sub(Script, environment):