[3.8] bpo-37206: Unrepresentable default values no longer represented as None. (GH-13933) (GH-16141)

In ArgumentClinic, value "NULL" should now be used only for unrepresentable default values
(like in the optional third parameter of getattr). "None" should be used if None is accepted
as argument and passing None has the same effect as not passing the argument at all.
(cherry picked from commit 279f44678c)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Python/bltinmodule.c

@@ -2147,7 +2147,7 @@ builtin_repr(PyObject *module, PyObject *obj)
 round as builtin_round
 
     number: object
-    ndigits: object = NULL
+    ndigits: object = None
 
 Round a number to a given precision in decimal digits.
 
@@ -2157,7 +2157,7 @@ the return value has the same type as the number.  ndigits may be negative.
 
 static PyObject *
 builtin_round_impl(PyObject *module, PyObject *number, PyObject *ndigits)
-/*[clinic end generated code: output=ff0d9dd176c02ede input=854bc3a217530c3d]*/
+/*[clinic end generated code: output=ff0d9dd176c02ede input=275678471d7aca15]*/
 {
     PyObject *round, *result;
 
@@ -2175,7 +2175,7 @@ builtin_round_impl(PyObject *module, PyObject *number, PyObject *ndigits)
         return NULL;
     }
 
-    if (ndigits == NULL || ndigits == Py_None)
+    if (ndigits == Py_None)
         result = _PyObject_CallNoArg(round);
     else
         result = PyObject_CallFunctionObjArgs(round, ndigits, NULL);