Enable null-safety in spring-security-oauth2-client

Closes gh-17819

config/src/test/java/org/springframework/security/config/oauth2/client/ClientRegistrationsBeanDefinitionParserTests.java

@@ -157,7 +157,7 @@ public class ClientRegistrationsBeanDefinitionParserTests {
 			.isEqualTo(ClientAuthenticationMethod.CLIENT_SECRET_BASIC);
 		assertThat(googleRegistration.getAuthorizationGrantType()).isEqualTo(AuthorizationGrantType.AUTHORIZATION_CODE);
 		assertThat(googleRegistration.getRedirectUri()).isEqualTo("{baseUrl}/{action}/oauth2/code/{registrationId}");
-		assertThat(googleRegistration.getScopes()).isNull();
+		assertThat(googleRegistration.getScopes()).isEmpty();
 		assertThat(googleRegistration.getClientName()).isEqualTo(serverUrl);
 		ProviderDetails googleProviderDetails = googleRegistration.getProviderDetails();
 		assertThat(googleProviderDetails).isNotNull();

config/src/test/kotlin/org/springframework/security/config/annotation/web/oauth2/login/AuthorizationEndpointDslTests.kt

@@ -73,13 +73,11 @@ class AuthorizationEndpointDslTests {
 
         companion object {
             val RESOLVER: OAuth2AuthorizationRequestResolver = object : OAuth2AuthorizationRequestResolver {
-                override fun resolve(
-                    request: HttpServletRequest?
-                ) = OAuth2AuthorizationRequest.authorizationCode().build()
+                override fun resolve(request: HttpServletRequest) =
+                    OAuth2AuthorizationRequest.authorizationCode().build()
 
-                override fun resolve(
-                    request: HttpServletRequest?, clientRegistrationId: String?
-                ) = OAuth2AuthorizationRequest.authorizationCode().build()
+                override fun resolve(request: HttpServletRequest, clientRegistrationId: String) =
+                    OAuth2AuthorizationRequest.authorizationCode().build()
             }
         }