AttributeStatement with absent Attributes

a pySAML2 SP seems to accept a Response with a missing Attribute Element as follow

````
        <saml:AttributeStatement>
        </saml:AttributeStatement>
````

even in this way
````
        <saml:AttributeStatement>
            <saml:Attribute Name="ingoalla" NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:basic"/>
        </saml:AttributeStatement>
````

