Polish ClientSecretBasicAuthenticationConverter

oauth2-authorization-server/src/main/java/org/springframework/security/oauth2/server/authorization/web/authentication/ClientSecretBasicAuthenticationConverter.java

@@ -25,6 +25,7 @@ import java.util.Map;
 import javax.servlet.http.HttpServletRequest;
 
 import org.springframework.http.HttpHeaders;
+import org.springframework.lang.Nullable;
 import org.springframework.security.core.Authentication;
 import org.springframework.security.oauth2.core.ClientAuthenticationMethod;
 import org.springframework.security.oauth2.core.OAuth2AuthenticationException;
@@ -48,6 +49,7 @@ import org.springframework.util.StringUtils;
  */
 public final class ClientSecretBasicAuthenticationConverter implements AuthenticationConverter {
 
+	@Nullable
 	@Override
 	public Authentication convert(HttpServletRequest request) {
 		String header = request.getHeader(HttpHeaders.AUTHORIZATION);