An invalid mapping is generated from an assembly containing no attributed classes.

I looked through the HbmSerializer+Serialize method and found no check for such situation which implies the following output:
<?xml version="1.0" encoding="utf-8" ?>

• <!-- Generated from NHibernate.Mapping.Attributes on 2009-10-09 09:55:38Z.
  -->
  <hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" />

and this causes NHibernate.Mapping exception when parsing this file.

The only problem is to decide what should be done in this situation (empty stream, exception, boolean return value or other?).