gh-87613: Pass NULL for an empty **kwargs in Argument Clinic

The parser allocated an empty dict for **kwargs when there were no
keyword arguments.  Pass NULL instead, like the caller does, and handle
NULL in the impls that used the dict unconditionally.

All **kwargs impls were audited: _csv and zoneinfo already accept NULL,
and the rest (structseq and SimpleNamespace `__replace__`,
_interpreters.new_config, atexit.register, SubElement) now check for it.
atexit stores None, which the run loop already maps back to NULL.

pyperf, PGO+LTO, `t = time.gmtime(0)`, `ns = SimpleNamespace(a=1)`:

| Benchmark                       | main   | this change           |
|---------------------------------|:------:|:---------------------:|
| struct_time.`__replace__`()     | 229 ns | 47.0 ns: 4.87x faster |
| SimpleNamespace.`__replace__`() | 72.2 ns | 65.5 ns: 1.10x faster |

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: cmaloney

Lib/test/test_clinic.py

@@ -5708,7 +5708,7 @@ def errmsg(name):
     def test_lone_kwds(self):
         with self.assertRaises(TypeError):
             ac_tester.lone_kwds(1, 2)
-        self.assertEqual(ac_tester.lone_kwds(), ({},))
+        self.assertEqual(ac_tester.lone_kwds(), (None,))
         self.assertEqual(ac_tester.lone_kwds(y='y'), ({'y': 'y'},))
         kwds = {'y': 'y', 'z': 'z'}
         self.assertEqual(ac_tester.lone_kwds(y='y', z='z'), (kwds,))
@@ -5721,7 +5721,7 @@ def test_kwds_with_pos_only(self):
             ac_tester.kwds_with_pos_only(y='y')
         with self.assertRaises(TypeError):
             ac_tester.kwds_with_pos_only(1, y='y')
-        self.assertEqual(ac_tester.kwds_with_pos_only(1, 2), (1, 2, {}))
+        self.assertEqual(ac_tester.kwds_with_pos_only(1, 2), (1, 2, None))
         self.assertEqual(ac_tester.kwds_with_pos_only(1, 2, y='y'), (1, 2, {'y': 'y'}))
         kwds = {'y': 'y', 'z': 'z'}
         self.assertEqual(ac_tester.kwds_with_pos_only(1, 2, y='y', z='z'), (1, 2, kwds))
@@ -5732,16 +5732,16 @@ def test_kwds_with_optional_pos_only(self):
             ac_tester.kwds_with_optional_pos_only()
         with self.assertRaises(TypeError):
             ac_tester.kwds_with_optional_pos_only(y='y')
-        self.assertEqual(ac_tester.kwds_with_optional_pos_only(1), (1, None, {}))
-        self.assertEqual(ac_tester.kwds_with_optional_pos_only(1, 2), (1, 2, {}))
+        self.assertEqual(ac_tester.kwds_with_optional_pos_only(1), (1, None, None))
+        self.assertEqual(ac_tester.kwds_with_optional_pos_only(1, 2), (1, 2, None))
         self.assertEqual(ac_tester.kwds_with_optional_pos_only(1, y='y'),
                          (1, None, {'y': 'y'}))
         self.assertEqual(ac_tester.kwds_with_optional_pos_only(1, 2, y='y'),
                          (1, 2, {'y': 'y'}))
 
     def test_kwds_with_stararg(self):
-        self.assertEqual(ac_tester.kwds_with_stararg(), ((), {}))
-        self.assertEqual(ac_tester.kwds_with_stararg(1, 2), ((1, 2), {}))
+        self.assertEqual(ac_tester.kwds_with_stararg(), ((), None))
+        self.assertEqual(ac_tester.kwds_with_stararg(1, 2), ((1, 2), None))
         self.assertEqual(ac_tester.kwds_with_stararg(y='y'), ((), {'y': 'y'}))
         args = (1, 2)
         kwds = {'y': 'y', 'z': 'z'}
@@ -5753,7 +5753,7 @@ def test_kwds_with_pos_only_and_stararg(self):
             ac_tester.kwds_with_pos_only_and_stararg()
         with self.assertRaises(TypeError):
             ac_tester.kwds_with_pos_only_and_stararg(y='y')
-        self.assertEqual(ac_tester.kwds_with_pos_only_and_stararg(1, 2), (1, 2, (), {}))
+        self.assertEqual(ac_tester.kwds_with_pos_only_and_stararg(1, 2), (1, 2, (), None))
         self.assertEqual(ac_tester.kwds_with_pos_only_and_stararg(1, 2, y='y'), (1, 2, (), {'y': 'y'}))
         args = ('lobster', 'thermidor')
         kwds = {'y': 'y', 'z': 'z'}

Modules/_elementtree.c

@@ -640,11 +640,11 @@ _elementtree_SubElement_impl(PyObject *module, PyObject *parent,
         attrib = PyDict_Copy(attrib);
         if (!attrib)
             return NULL;
-        if (PyDict_Update(attrib, extra) < 0) {
+        if (extra && PyDict_Update(attrib, extra) < 0) {
             Py_DECREF(attrib);
             return NULL;
         }
-    } else if (PyDict_GET_SIZE(extra)) {
+    } else if (extra && PyDict_GET_SIZE(extra)) {
         /* have keyword args */
         attrib = get_attrib_from_keywords(extra);
         if (!attrib)

Modules/_interpretersmodule.c

@@ -828,7 +828,7 @@ _interpreters_new_config_impl(PyObject *module, const char *name,
         return NULL;
     }
 
-    if (PyDict_GET_SIZE(overrides) > 0) {
+    if (overrides != NULL && PyDict_GET_SIZE(overrides) > 0) {
         if (_PyInterpreterConfig_UpdateFromDict(&config, overrides) < 0) {
             return NULL;
         }

Modules/atexitmodule.c

@@ -191,7 +191,8 @@ atexit_register_impl(PyObject *module, PyObject *func, PyObject *args,
                 "the first argument must be callable");
         return NULL;
     }
-    PyObject *callback = PyTuple_Pack(3, func, args, kwargs);
+    PyObject *callback = PyTuple_Pack(3, func, args,
+                                      kwargs ? kwargs : Py_None);
     if (callback == NULL)
     {
         return NULL;