bugfix: In case an empty SubjectConfirmation is received, an appropriate error must be thrown.

This resolves #530. There are two problems here:

- When only one SubjectConfirmation is received and it is empty, an error should be thrown. However, the error would be a not very descriptive message warning about access to a non-property in a null object. Something more descriptive should be in place.
- Additionally, in PHP 7.0 this is an error and not an exception, and then the code continues to execute, effectively allowing assertions without a proper SubjectConfirmation element. This is wrong according to the standard.