要解决Apache Jackrabbit WebDAV成功上传XML文件但下载的文件为空的问题，可以尝试以下解决方法：

1. 确保上传的XML文件内容不为空：在上传XML文件之前，可以使用打印语句或调试器检查文件内容是否为空。如果文件内容为空，可以修改代码或处理逻辑，确保正确读取XML文件内容。

2. 检查文件路径和名称：确保下载文件时使用的路径和文件名与上传时完全匹配。文件路径和名称的大小写也需要注意。

3. 确保文件类型正确：在上传和下载XML文件时，需要设置正确的Content-Type。确保在上传时设置Content-Type为"application/xml"，并在下载时设置正确的Accept头部或Content-Type为"application/xml"。

以下是一个使用Apache HttpClient库上传和下载XML文件的示例代码：

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.entity.ContentType;
import org.apache.http.entity.FileEntity;
import org.apache.http.impl.client.DefaultHttpClient;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;

public class WebDAVExample {

    private static final String UPLOAD_URL = "http://example.com/webdav/upload";
    private static final String DOWNLOAD_URL = "http://example.com/webdav/download/myfile.xml";

    public static void main(String[] args) {
        // 上传XML文件
        File xmlFile = new File("path/to/myfile.xml");
        uploadFile(xmlFile);

        // 下载XML文件
        downloadFile("path/to/downloaded_file.xml");
    }

    private static void uploadFile(File file) {
        HttpClient httpClient = new DefaultHttpClient();
        HttpPost httpPost = new HttpPost(UPLOAD_URL);

        FileEntity fileEntity = new FileEntity(file, ContentType.APPLICATION_XML);
        httpPost.setEntity(fileEntity);

        try {
            HttpResponse response = httpClient.execute(httpPost);
            // 处理上传结果
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            httpClient.getConnectionManager().shutdown();
        }
    }

    private static void downloadFile(String downloadPath) {
        HttpClient httpClient = new DefaultHttpClient();
        HttpGet httpGet = new HttpGet(DOWNLOAD_URL);

        try {
            HttpResponse response = httpClient.execute(httpGet);
            HttpEntity entity = response.getEntity();

            if (entity != null) {
                try (InputStream inputStream = entity.getContent();
                     FileOutputStream outputStream = new FileOutputStream(downloadPath)) {

                    byte[] buffer = new byte[1024];
                    int bytesRead;
                    while ((bytesRead = inputStream.read(buffer)) != -1) {
                        outputStream.write(buffer, 0, bytesRead);
                    }
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            httpClient.getConnectionManager().shutdown();
        }
    }
}

请根据实际情况修改示例代码中的URL和文件路径。这是一个基本示例，你可以根据自己的需求进行适当调整和优化。