如何使用php rest api修复reactjs前端数据库中的空插入？

<?php
// 连接数据库
$servername = "localhost";
$username = "your_username";
$password = "your_password";
$dbname = "your_database";

$conn = new mysqli($servername, $username, $password, $dbname);
if ($conn->connect_error) {
    die("数据库连接失败: " . $conn->connect_error);
}

// 处理REST API请求
if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    $data = json_decode(file_get_contents('php://input'), true);

    // 检查插入数据是否为空
    if (empty($data)) {
        // 返回错误消息
        echo json_encode(array("message" => "插入数据不能为空"));
        http_response_code(400);
    } else {
        // 执行插入操作
        // 这里假设你的表名为"users"，并且有"username"和"email"字段
        $username = $data['username'];
        $email = $data['email'];

        $sql = "INSERT INTO users (username, email) VALUES ('$username', '$email')";
        if ($conn->query($sql) === TRUE) {
            // 返回成功消息
            echo json_encode(array("message" => "插入成功"));
            http_response_code(200);
        } else {
            // 返回错误消息
            echo json_encode(array("message" => "插入失败：" . $conn->error));
            http_response_code(500);
        }
    }
}

// 关闭数据库连接
$conn->close();
?>

const data = {
    username: 'John',
    email: 'john@example.com'
};

fetch('http://your_domain.com/api/insert.php', {
    method: 'POST',
    headers: {
        'Content-Type': 'application/json',
    },
    body: JSON.stringify(data),
})
.then(response => response.json())
.then(data => {
    console.log(data);
})
.catch(error => {
    console.error('Error:', error);
});