NPE in BaseOpenSamlAuthenticationProvider

[fournm]

Describe the bug
A clear and concise description of what the bug is.

SAML response parsing is incorrect in 6.4.5's BaseOpenSamlAuthenticationProvider and results in a NullPointerException

Response has the Issuer field correctly marked as @Nullable, but the process method treats issuer in Response as non-null, sometimes resulting in an error while creating a logging statement.

To Reproduce
Steps to reproduce the behavior.

1. Provide SAML response with Issuer in Assertion, not as a direct child to the saml2p:Response element
2. get nullpointerexception on line 317 of BaseOpenSamlAuthenticationProvider

Expected behavior
A clear and concise description of what you expected to happen.

No null pointer exception for responses that valid against the schema. I would expect to see issuer = null instead.