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/test/java/org/elasticsearch/test/SecurityIntegTestCase.java
* {@link org.elasticsearch.test.ESIntegTestCase.Scope#SUITE} or {@link org.elasticsearch.test.ESIntegTestCase.Scope#TEST}
     */
    protected String configUsers() {
        return SECURITY_DEFAULT_SETTINGS.configUsers();
    }
    /**
     * Allows to override the users_roles config file when the {@link org.elasticsearch.test.ESIntegTestCase.ClusterScope} is set to
     * {@link org.elasticsearch.test.ESIntegTestCase.Scope#SUITE} or {@link org.elasticsearch.test.ESIntegTestCase.Scope#TEST}
     */
    protected String configUsersRoles() {
        return SECURITY_DEFAULT_SETTINGS.configUsersRoles();
    }
    /**
     * Allows to override the roles config file when the {@link org.elasticsearch.test.ESIntegTestCase.ClusterScope} is set to
     * {@link org.elasticsearch.test.ESIntegTestCase.Scope#SUITE} or {@link org.elasticsearch.test.ESIntegTestCase.Scope#TEST}
     */
    protected String configRoles() {
        return SECURITY_DEFAULT_SETTINGS.configRoles();
    }
    protected String configOperatorUsers() {
        return SECURITY_DEFAULT_SETTINGS.configOperatorUsers();
    }
    /**
     * Allows to override the node client username (used while sending requests to the test cluster) when the
     * {@link org.elasticsearch.test.ESIntegTestCase.ClusterScope} is set to
     * {@link org.elasticsearch.test.ESIntegTestCase.Scope#SUITE} or {@link org.elasticsearch.test.ESIntegTestCase.Scope#TEST}
     */
    protected String nodeClientUsername() {
        return SECURITY_DEFAULT_SETTINGS.nodeClientUsername();
    }