fixed problem with shell introduced in recent commit

VERSION

@@ -1 +1 @@
-Version 2.00.0 (2012-05-28 16:19:00) dev
+Version 2.00.0 (2012-05-28 16:30:27) dev

gluon/shell.py

@@ -205,7 +205,7 @@ def run(
         exec ('print %s()' % f, _env)
         return
 
-    _env.update(exec_pythonrc())
+    _env.update(exec_pythonrc() or {})
     if startfile:
         try:
             execfile(startfile, _env)