public String documentationOf(String key) {
        ConfigDef.ConfigKey configKey = definition.configKeys().get(key);
        if (configKey == null)
            return null;
        return configKey.documentation;
    }
@Test
    public void testDocumentationOfExpectNull() {
        Properties props = new Properties();
        TestIndirectConfigResolution config = new TestIndirectConfigResolution(props);

        assertNull(config.documentationOf("xyz"));
    }