[FIX] Hr_expense: 'NoneType' object does not support item assignment

lp bug: https://launchpad.net/bugs/691290 fixed

bzr revid: mra@mra-laptop-20101217054631-vkm93baylg7oggk1

addons/hr_expense/hr_expense.py

@@ -240,6 +240,8 @@ class hr_expense_line(osv.osv):
     _order = "sequence, date_value desc"
 
     def onchange_product_id(self, cr, uid, ids, product_id, uom_id, employee_id, context=None):
+        if context is None:
+            context = {}
         res = {}
         if product_id:
             product = self.pool.get('product.product').browse(cr, uid, product_id, context=context)