[FIX] base: fixing fix, attachment file size and None

In commit 44f2c8d54 we unified the return value of the function to int,
but it seems the returned size could be None which is not a valid input
of the int() built-in function.

openerp/addons/base/ir/ir_attachment.py

@@ -133,7 +133,7 @@ class ir_attachment(osv.osv):
             else:
                 result[attach.id] = attach.db_datas
                 if bin_size:
-                    result[attach.id] = int(result[attach.id])
+                    result[attach.id] = int(result[attach.id] or 0)
 
         return result