empty assertions

["Tom Scavo" <trscavo@gmail.com>]

In response to an attribute query, if no attributes for the principal
are available, it seems the IdP should return an empty response.  In
SAML V2.0, does it make sense to return an empty assertion instead?
(This is legal in SAML V2.0, right?)

In general, what are the uses of an empty assertion?

Thanks,
Tom Scavo
NCSA