PyCLIF callback_test.py: Add testPassNoneAsCallback

This CL was used in a PyCLIF-pybind11 TGP (cl/667456010, 2024-08-26):

http://tap/OCL:667456010:BASE:667610796:1724689781533:d2626456:

```
    571 Failing Targets
    147 Broken Targets
```

For this to work, a pybind11clif fix is needed (merged already):

* google/pybind11clif#30155

See also: https://b.corp.google.com/issues/287289622#comment75

PiperOrigin-RevId: 668995414

clif/pybind11/function_lib.py

@@ -402,47 +402,55 @@ def generate_py_args(func_decl: ast_pb2.FuncDecl,
   return ', '.join(params_list)
 
 
+def _get_noconvert(param: ast_pb2.ParamDecl):
+  if param.type.HasField('callable'):
+    return '.noconvert()'
+  return ''
+
+
 def _generate_py_arg_with_default(
     param: ast_pb2.ParamDecl, return_value_policy_pack: str
 ) -> str:
   """Generate `pybind11::arg` for parameters with default value."""
+  noconvert = _get_noconvert(param)
   if return_value_policy_pack:
     if param.default_value == 'nullptr':
       return (
-          f'pybind11::arg("{param.name.cpp_name}")'
-          f'.policies({return_value_policy_pack}) = {param.default_value}'
+          f'pybind11::arg("{param.name.cpp_name}"){noconvert}'
+          f'.policies({return_value_policy_pack}) = '
+          f'{param.default_value}'
       )
     else:
       return (
-          f'pybind11::arg("{param.name.cpp_name}")'
+          f'pybind11::arg("{param.name.cpp_name}"){noconvert}'
           f'.policies({return_value_policy_pack}) = '
           f'static_cast<{param.type.cpp_type}>({param.default_value})'
       )
   else:
     if param.default_value == 'nullptr':
       if param.type.cpp_type == _CPP_TYPE_PYOBJECT_PTR_FROM_MATCHER:
         return (
-            f'pybind11::arg("{param.name.cpp_name}") ='
-            ' pybind11::nullptr_default_arg()'
+            f'pybind11::arg("{param.name.cpp_name}"){noconvert} = '
+            'pybind11::nullptr_default_arg()'
         )
-      return f'pybind11::arg("{param.name.cpp_name}") = {param.default_value}'
+      return (f'pybind11::arg("{param.name.cpp_name}"){noconvert} = '
+              f'{param.default_value}')
     else:
       return (
-          f'pybind11::arg("{param.name.cpp_name}") = '
+          f'pybind11::arg("{param.name.cpp_name}"){noconvert} = '
           f'static_cast<{param.type.cpp_type}>({param.default_value})'
       )
 
 
 def _generate_py_arg_without_default(
     param: ast_pb2.ParamDecl, return_value_policy_pack: str
 ) -> str:
+  noconvert = _get_noconvert(param)
   if return_value_policy_pack:
-    return (
-        f'pybind11::arg("{param.name.cpp_name}")'
-        f'.policies({return_value_policy_pack})'
-    )
+    return (f'pybind11::arg("{param.name.cpp_name}"){noconvert}'
+            f'.policies({return_value_policy_pack})')
   else:
-    return f'pybind11::arg("{param.name.cpp_name}")'
+    return f'pybind11::arg("{param.name.cpp_name}{noconvert}")'
 
 
 def _generate_return_value_policy_pack_for_py_arg(

clif/testing/python/callback_test.py

@@ -112,6 +112,10 @@ def testBytesCallback(self):
     self.assertIsInstance(res, bytes)
     self.assertEqual(res, b'abcd')
 
+  def testPassNoneAsCallback(self):
+    with self.assertRaises(TypeError):
+      callback.LambdaCallback(None)
+
   def testPyObjectCallback(self):
     res = callback.PyObjectCallback(StringCallback)
     self.assertIsInstance(res, str)