Only validate the Destination element of an incoming SAML Response
if the SAML Response is signed.
The standard [1] - 3.5.5.2 and [2] - 3.2.2 does mention that the
Destination element is optional and should only be verified when
the SAML Response is signed. Some Identity Provider implementations
are known to not set a Destination XML Attribute in their SAML
responses when those are not signed, so this change also aims to
enhance interoperability.

[1] https://docs.oasis-open.org/security/saml/v2.0/saml-bindings-2.0-os.pdf
[2] https://docs.oasis-open.org/security/saml/v2.0/saml-core-2.0-os.pdf