private void ensureInternalClient() {
        checkState(client == null,
                   "This method is allowed only when created with a connection string.");
    }
Returns {@code true} if this builder is created with
{@link #AbstractCuratorFrameworkBuilder(CuratorFramework, String)}.
zookeeper3/src/main/java/com/linecorp/armeria/common/zookeeper/AbstractCuratorFrameworkBuilder.java