gh-106307: Fix PyMapping_GetOptionalItemString() (GH-108797)

The resulting pointer was not set to NULL if the creation of a temporary
string object was failed.

The tests were also missed due to oversight.

Objects/abstract.c

@@ -2393,11 +2393,13 @@ int
 PyMapping_GetOptionalItemString(PyObject *obj, const char *key, PyObject **result)
 {
     if (key == NULL) {
+        *result = NULL;
         null_error();
         return -1;
     }
     PyObject *okey = PyUnicode_FromString(key);
     if (okey == NULL) {
+        *result = NULL;
         return -1;
     }
     int rc = PyMapping_GetOptionalItem(obj, okey, result);