getIdpBinding - handle no bindings specified

[claylehman]

I am trying to hookup SAML with PingFederate.

The metadata.xml file generated by their system does NOT include any md:SingleSignOnService... data.

It would be nice for my users if they could just export the metadata.xml and use it as is in my system instead of having to manually add an entry for md:SingleSignOnService

I would like to build up a PR to make getIdpBinding() to return NULL if there were no md:SingleSignOnService at all, vs there were some, but none matched. Then downstream, I will make the places using the results of it handle null by defaulting to the value from assertionConsumerServiceUrl - sort of like the opposite of what you do if its Okta