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
/**
     * Allows to override the node client password (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 SecureString nodeClientPassword() {
        return SECURITY_DEFAULT_SETTINGS.nodeClientPassword();
    }
    /**
     * Allows to control whether ssl key information is auto generated or not on the transport layer
     */
    protected boolean transportSSLEnabled() {
        return randomBoolean();
    }
    protected int maxNumberOfNodes() {
        return defaultMaxNumberOfNodes();
    }
    private class CustomSecuritySettingsSource extends SecuritySettingsSource {
        private CustomSecuritySettingsSource(boolean sslEnabled, Path configDir, Scope scope) {
            super(sslEnabled, configDir, scope);
        }
        @Override
        protected String configUsers() {
            return SecurityIntegTestCase.this.configUsers();
        }
        @Override
        protected String configUsersRoles() {
            return SecurityIntegTestCase.this.configUsersRoles();
        }
        @Override