[3.15] gh-155621: Fix unrepresentable defaults in socket.sendmsg() signature (GH-155623) (GH-155802)

(cherry picked from commit e5556cd)

Co-authored-by: stevens <lipengyu@kylinos.cn>

Author: aisk

Modules/clinic/socketmodule.c.h

@@ -4925,9 +4925,9 @@ sock_sendmsg_impl(PySocketSockObject *s, void *data)
 _socket.socket.sendmsg
     self as s: self(type="PySocketSockObject *")
     buffers as data_arg: object
-    ancdata as cmsg_arg: object = NULL
+    ancdata as cmsg_arg: object(c_default="NULL") = ()
     flags: int = 0
-    address as addr_arg: object = NULL
+    address as addr_arg: object(c_default="NULL") = None
     /
 
 Send normal and ancillary data to the socket.
@@ -4951,7 +4951,7 @@ static PyObject *
 _socket_socket_sendmsg_impl(PySocketSockObject *s, PyObject *data_arg,
                             PyObject *cmsg_arg, int flags,
                             PyObject *addr_arg)
-/*[clinic end generated code: output=3b4cb1110644ce39 input=8ae408971a3aa329]*/
+/*[clinic end generated code: output=3b4cb1110644ce39 input=5fdf9b49bed3848b]*/
 
 {
     Py_ssize_t i, ndatabufs = 0, ncmsgs, ncmsgbufs = 0;