fixed a bug raised by Raymond Wiker, docSetRootElement() should not raise

* python/generator.py: fixed a bug raised by Raymond Wiker,
  docSetRootElement() should not raise an exception if the
  return is None
Daniel
diff --git a/python/generator.py b/python/generator.py
index 2d1de5a..9adbfbe 100755
--- a/python/generator.py
+++ b/python/generator.py
@@ -632,6 +632,7 @@
 functions_noexcept = {
     "xmlHasProp": 1,
     "xmlHasNsProp": 1,
+    "xmlDocSetRootElement": 1,
 }
 
 reference_keepers = {