fix upb python: DO NOT create empty optional field after sorting empty repeated field

PiperOrigin-RevId: 618350866

python/google/protobuf/internal/message_test.py

@@ -549,6 +549,15 @@ class MessageTest(unittest.TestCase):
     self.assertEqual([4, 3, 2, 1],
                      [m.bb for m in msg.repeated_nested_message[::-1]])
 
+  def testSortEmptyRepeated(self, message_module):
+    message = message_module.NestedTestAllTypes()
+    self.assertFalse(message.HasField('child'))
+    self.assertFalse(message.HasField('payload'))
+    message.child.repeated_child.sort()
+    message.payload.repeated_int32.sort()
+    self.assertFalse(message.HasField('child'))
+    self.assertFalse(message.HasField('payload'))
+
   def testSortingRepeatedScalarFieldsDefaultComparator(self, message_module):
     """Check some different types with the default comparator."""
     message = message_module.TestAllTypes()

python/repeated.c

@@ -473,11 +473,14 @@ static PyObject* PyUpb_RepeatedContainer_Sort(PyObject* pself, PyObject* args,
     }
   }
 
+  if (PyUpb_RepeatedContainer_Length(pself) == 0) Py_RETURN_NONE;
+
   PyObject* ret = NULL;
   PyObject* full_slice = NULL;
   PyObject* list = NULL;
   PyObject* m = NULL;
   PyObject* res = NULL;
+
   if ((full_slice = PySlice_New(NULL, NULL, NULL)) &&
       (list = PyUpb_RepeatedContainer_Subscript(pself, full_slice)) &&
       (m = PyObject_GetAttrString(list, "sort")) &&