Require non-null authenticationRequest

Closes gh-18880

Signed-off-by: Josh Cummings <3627351+jzheaux@users.noreply.github.com>

saml2/saml2-service-provider/src/main/java/org/springframework/security/saml2/provider/service/web/CacheSaml2AuthenticationRequestRepository.java

@@ -52,6 +52,7 @@ public final class CacheSaml2AuthenticationRequestRepository
 	@Override
 	public void saveAuthenticationRequest(AbstractSaml2AuthenticationRequest authenticationRequest,
 			HttpServletRequest request, HttpServletResponse response) {
+		Assert.notNull(authenticationRequest, "authenticationRequest must not be null");
 		String relayState = request.getParameter(Saml2ParameterNames.RELAY_STATE);
 		Assert.notNull(relayState, "relayState must not be null");
 		this.cache.put(relayState, authenticationRequest);