Avoid calling PyUnicode_FromUnicode() in Py3.

Closes https://github.com/cython/cython/pull/3677

Cython/Utility/StringTools.c

@@ -443,6 +443,7 @@ static CYTHON_INLINE PyObject* __Pyx_decode_c_string(
 /////////////// decode_c_string ///////////////
 //@requires: IncludeStringH
 //@requires: decode_c_string_utf16
+//@substitute: naming
 
 /* duplicate code to avoid calling strlen() if start >= 0 and stop >= 0 */
 static CYTHON_INLINE PyObject* __Pyx_decode_c_string(
@@ -467,7 +468,7 @@ static CYTHON_INLINE PyObject* __Pyx_decode_c_string(
             stop += length;
     }
     if (unlikely(stop <= start))
-        return PyUnicode_FromUnicode(NULL, 0);
+        return __Pyx_NewRef($empty_unicode);
     length = stop - start;
     cstring += start;
     if (decode_func) {
@@ -486,6 +487,7 @@ static CYTHON_INLINE PyObject* __Pyx_decode_c_bytes(
 
 /////////////// decode_c_bytes ///////////////
 //@requires: decode_c_string_utf16
+//@substitute: naming
 
 static CYTHON_INLINE PyObject* __Pyx_decode_c_bytes(
          const char* cstring, Py_ssize_t length, Py_ssize_t start, Py_ssize_t stop,
@@ -503,7 +505,7 @@ static CYTHON_INLINE PyObject* __Pyx_decode_c_bytes(
     if (stop > length)
         stop = length;
     if (unlikely(stop <= start))
-        return PyUnicode_FromUnicode(NULL, 0);
+        return __Pyx_NewRef($empty_unicode);
     length = stop - start;
     cstring += start;
     if (decode_func) {