python: return None instead of bytearray() for zero length variable b…

…yte buffers

src/swig_python/python_extra.py_in

@@ -5,6 +5,8 @@ def _wrap_bin(fn, length, resize=False):
     """ Wrap functions that take an output buffer to create/return it """
     def wrapped(*args):
         n = length(*args) if callable(length) else length
+        if not n:
+            return None
         buf = bytearray(n)
         ret = fn(*list(args)+[buf])
         if resize: