Metadata: Ensure SP DS endpoints only rendered if 1+ functioning.

Previously when an SP disabled all DS endpoints (but did not delete)
we'd be left with the following:

    <Extensions/>

Which is invalid against schema checks.

This change ensures at least one discovery service endpoint is
functioning before adding this element to the builder.

Correctly functining discovery service endpoints result it

    <Extensions>
      <dsr:DiscoveryResponse xmlns:dsr="urn:oasis:names:tc:SAML:profiles:SSO:idp-discovery-protocol" Binding="urn:oasis:names:tc:SAML:profiles:SSO:idp-discovery-protocol" Location="https://test.example.com/Shibboleth.sso/DS" index="1" isDefault="true" />
    </Extensions>
Merge pull request #129 from ausaccessfed/feature/#129-invalid-metadata-when-all-discovery-invalid

FR generated invalid metadata if all SP DiscoveryService endpoints are marked as inactive