Handle attributes with empty values

This change allows the `Response#attributes` method to handle attribute elements with empty values. The example response that was breaking the current build looks something like this:

`````
<saml:Attribute Name="email" NameFormat="urn:oasis:names:tc:SAML:2.0:attrname-format:uri">
   <saml:AttributeValue/>
</saml:Attribute>
`````
It may be that empty attributes are illegal in SAML and this is unnecessary? Either way, please let me know. Thanks!