We pass `null` for the class name here:

https://github.com/elastic/elasticsearch/blob/f086dd18ddc705bf84a1ff99d35196ec2498fd52/server/src/main/java/org/elasticsearch/plugins/PluginDescriptor.java#L261

However the class name is mandatory for the transport protocol today:

https://github.com/elastic/elasticsearch/blob/f086dd18ddc705bf84a1ff99d35196ec2498fd52/server/src/main/java/org/elasticsearch/plugins/PluginDescriptor.java#L156

This prevents various APIs from working in a cluster with stable plugins installed.
x-pack/plugin/security/src/internalClusterTest/java/org/elasticsearch/test/SecuritySingleNodeTestCase.java
@Override
        protected String configRoles() {
            return SecuritySingleNodeTestCase.this.configRoles();
        }
        @Override
        protected String configOperatorUsers() {
            return SecuritySingleNodeTestCase.this.configOperatorUsers();
        }
        @Override
        protected String configServiceTokens() {
            return SecuritySingleNodeTestCase.this.configServiceTokens();
        }
        @Override
        protected String nodeClientUsername() {
            return SecuritySingleNodeTestCase.this.nodeClientUsername();
        }
        @Override
        protected SecureString nodeClientPassword() {
            return SecuritySingleNodeTestCase.this.nodeClientPassword();
        }
    }
    @Override
    public Client wrapClient(final Client client) {
        Map<String, String> headers = Collections.singletonMap(
            "Authorization",
            basicAuthHeaderValue(nodeClientUsername(), nodeClientPassword())
        );