marshal: optimize parsing of empty Unicode strings

Don't create a temporary buffer of zeroy byte nor call r_string() if the length
is zero, create directly the empty string.

Python/marshal.c

@@ -979,6 +979,7 @@ r_object(RFILE *p)
             retval = NULL;
             break;
         }
+        if (n != 0) {
             buffer = PyMem_NEW(char, n);
             if (buffer == NULL) {
                 retval = PyErr_NoMemory();
@@ -993,6 +994,10 @@ r_object(RFILE *p)
             }
             v = PyUnicode_DecodeUTF8(buffer, n, "surrogatepass");
             PyMem_DEL(buffer);
+        }
+        else {
+            v = PyUnicode_New(0, 0);
+        }
         if (type == TYPE_INTERNED)
             PyUnicode_InternInPlace(&v);
         retval = v;