String getTopicConfig(String topicName, String configKey) throws NoSuchElementException {
        Map<String, String> map = configData.get(new ConfigResource(Type.TOPIC, topicName));
        if (map == null || !map.containsKey(configKey)) {
            Map<String, ConfigEntry> effectiveConfigMap = computeEffectiveTopicConfigs(Collections.emptyMap());
            if (!effectiveConfigMap.containsKey(configKey)) {
                return null;
            }
            return effectiveConfigMap.get(configKey).value();
        }
        return map.get(configKey);
    }
Get the config value for the give topic and give config key.
If the config value is not found, return null.

@param topicName            The topic name for the config.
@param configKey            The key for the config.
metadata/src/main/java/org/apache/kafka/controller/ConfigurationControlManager.java