401 on SAML logout if asserting party does not support SLO

Describe the bug
When I POST to the SAML logoutUrl, a 401 response is returned.

To Reproduce

.logout(logout -> logout.invalidateHttpSession(false))  // issue #14853
.saml2Login(saml -> saml
    .authenticationRequestUri("/saml/authenticate/{registrationId}")
    .loginPage("/saml/discovery")
    .loginProcessingUrl("/saml/SSO")
)
.saml2Logout(saml -> saml
    .logoutUrl("/saml/logout")
    .logoutRequest(request -> request.logoutUrl("/saml/SingleLogout"))
    .logoutResponse(response -> response.logoutUrl("/saml/SingleLogout"))
)

14:15:21.139 [XNIO-1 task-2] DEBUG FilterChainProxy - Securing POST /saml/logout
14:15:21.144 [XNIO-1 task-2] DEBUG HttpSessionSecurityContextRepository - Retrieved SecurityContextImpl [Authentication=Saml2Authentication [Principal=User(...), Credentials=[PROTECTED], Authenticated=true, Details=WebAuthenticationDetails [RemoteIpAddress=127.0.0.1, SessionId=d39d3aab-9fa4-47e1-9e08-f83036c405e8], Granted Authorities=[ROLE_USER, ROLE_ADMIN]]]
14:15:21.144 [XNIO-1 task-2] DEBUG SecurityContextPersistenceFilter - Set SecurityContextHolder to SecurityContextImpl [Authentication=Saml2Authentication [Principal=User(...), Credentials=[PROTECTED], Authenticated=true, Details=WebAuthenticationDetails [RemoteIpAddress=127.0.0.1, SessionId=d39d3aab-9fa4-47e1-9e08-f83036c405e8], Granted Authorities=[ROLE_USER, ROLE_ADMIN]]]
14:15:21.144 [XNIO-1 task-2] DEBUG Saml2LogoutConfigurer$Saml2RelyingPartyInitiatedLogoutFilter - Logging out [Saml2Authentication [Principal=User(...), Credentials=[PROTECTED], Authenticated=true, Details=WebAuthenticationDetails [RemoteIpAddress=127.0.0.1, SessionId=d39d3aab-9fa4-47e1-9e08-f83036c405e8], Granted Authorities=[ROLE_USER, ROLE_ADMIN]]]
14:15:21.144 [XNIO-1 task-2] DEBUG HttpSessionSecurityContextRepository - Did not store empty SecurityContext
14:15:21.145 [XNIO-1 task-2] TRACE OpenSamlLogoutRequestResolver - Attempting to resolve registrationId from Saml2Authentication [Principal=User(...), Credentials=[PROTECTED], Authenticated=true, Details=WebAuthenticationDetails [RemoteIpAddress=127.0.0.1, SessionId=d39d3aab-9fa4-47e1-9e08-f83036c405e8], Granted Authorities=[ROLE_USER, ROLE_ADMIN]]
14:15:21.146 [XNIO-1 task-2] TRACE Saml2RelyingPartyInitiatedLogoutSuccessHandler - Returning 401 since no logout request generated
14:15:21.147 [XNIO-1 task-2] DEBUG HttpSessionSecurityContextRepository - Did not store empty SecurityContext
14:15:21.147 [XNIO-1 task-2] DEBUG SecurityContextPersistenceFilter - Cleared SecurityContextHolder to complete request

Expected behavior
It should redirect to / after logout, regardless of whether the asserting party was sent an SLO request.

Additional
The OpenSamlLogoutRequestResolver returned null here:

if (registration.getAssertingPartyDetails().getSingleLogoutServiceLocation() == null) {
    return null;
}

I'm guessing the intent is that the 401 triggers a redirect back to /login, but it should use the same mechanism as the regular logout in case someone is not using the default configuration.

Start with OpenSamlLogoutRequestResolver and Saml2RelyingPartyInitiatedLogoutSuccessHandler, tracing the path when no asserting-party logout service location produces a null logout request. Verify the logout flow redirects to / after session logout even when no SLO request is sent, and cover the behavior with the relevant SAML logout tests.