gh-90699: Use _Py_STR(empty) instead of PyUnicode_New(0, 0) for BUILD_STRING (GH-91476)

Author: sweeneyde

Python/ceval.c

@@ -3152,12 +3152,8 @@ _PyEval_EvalFrameDefault(PyThreadState *tstate, _PyInterpreterFrame *frame, int
 
         TARGET(BUILD_STRING) {
             PyObject *str;
-            PyObject *empty = PyUnicode_New(0, 0);
-            if (empty == NULL) {
-                goto error;
-            }
-            str = _PyUnicode_JoinArray(empty, stack_pointer - oparg, oparg);
-            Py_DECREF(empty);
+            str = _PyUnicode_JoinArray(&_Py_STR(empty),
+                                       stack_pointer - oparg, oparg);
             if (str == NULL)
                 goto error;
             while (--oparg >= 0) {