Avoid useless C-API call to get a constant.

Cython/Utility/StringTools.c

@@ -595,6 +595,7 @@ static CYTHON_INLINE PyObject* __Pyx_PyUnicode_Substring(
             PyObject* text, Py_ssize_t start, Py_ssize_t stop);
 
 /////////////// PyUnicode_Substring ///////////////
+//@substitute: naming
 
 static CYTHON_INLINE PyObject* __Pyx_PyUnicode_Substring(
             PyObject* text, Py_ssize_t start, Py_ssize_t stop) {
@@ -611,7 +612,7 @@ static CYTHON_INLINE PyObject* __Pyx_PyUnicode_Substring(
     else if (stop > length)
         stop = length;
     if (stop <= start)
-        return PyUnicode_FromUnicode(NULL, 0);
+        return __Pyx_NewRef($empty_unicode);
 #if CYTHON_PEP393_ENABLED
     return PyUnicode_FromKindAndData(PyUnicode_KIND(text),
         PyUnicode_1BYTE_DATA(text) + start*PyUnicode_KIND(text), stop-start);