Fix my last change on PyUnicode_Join(): don't process separator if len==1

Objects/unicodeobject.c

@@ -9145,13 +9145,16 @@ PyUnicode_Join(PyObject *separator, PyObject *seq)
 
     /* If singleton sequence with an exact Unicode, return that. */
     items = PySequence_Fast_ITEMS(fseq);
-    if (seqlen == 1 && PyUnicode_CheckExact(items[0])) {
+    if (seqlen == 1) {
+        if (PyUnicode_CheckExact(items[0])) {
             res = items[0];
             Py_INCREF(res);
             Py_DECREF(fseq);
             return res;
         }
-
+        sep = NULL;
+    }
+    else {
         /* Set up sep and seplen */
         if (separator == NULL) {
             /* fall back to a blank space separator */
@@ -9177,6 +9180,7 @@ PyUnicode_Join(PyObject *separator, PyObject *seq)
                above case of a blank separator */
             Py_INCREF(sep);
         }
+    }
 
     /* There are at least two things to join, or else we have a subclass
      * of str in the sequence.