Spring Security OidcBackChannelLogoutTokenValidator should not throw an NPE when issuer is missing

Related to https://github.com/spring-projects/spring-boot/issues/42172#issue-2510291998, when ClientRegistration does not have an issuerUri configured, it should give a better error than an NPE.

This needs to be addressed for both the imperative and reactive versions of the class.

Please base this change off the 6.2.x branch so it can be forward-ported when merged.

Comment From: wapkch

Hi @jzheaux , may i work on this?

Comment From: jzheaux

Yes, please. :) Thanks for offering, the issue is yours.