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.
server/src/test/java/org/elasticsearch/plugins/PluginsServiceTests.java
Path mypluginDir = pluginsDir.resolve("myplugin");
        PluginTestUtil.writePluginProperties(
            mypluginDir,
            "description",
            "whatever",
            "name",
            "myplugin",
            "version",
            "1.0.0",
            "elasticsearch.version",
            Version.CURRENT.toString(),
            "java.version",
            System.getProperty("java.specification.version"),
            "extended.plugins",
            "nonextensible",
            "classname",
            "test.DummyPlugin"
        );
        try (InputStream jar = PluginsServiceTests.class.getResourceAsStream("dummy-plugin.jar")) {
            Files.copy(jar, mypluginDir.resolve("plugin.jar"));
        }
        Path nonextensibleDir = pluginsDir.resolve("nonextensible");
        PluginTestUtil.writePluginProperties(
            nonextensibleDir,
            "description",
            "whatever",
            "name",
            "nonextensible",
            "version",
            "1.0.0",