Describe the bug

The options.description parameter is documented as optional, and the atom and json formats handle this by omitting this optional field in their output. The RSS2 format requires a <description> element and so we can't omit it. But, combined with the fact that the Feed class takes an optional description, means we serialize <description>undefined</description> which then ends up on-screen in feed readers.

To Reproduce
Steps to reproduce the behavior:

• new Feed() with description unset, undefined, or empty string.
• feed.rss2()

Example:

• new Feed https://github.com/nodejs/nodejs.org/blob/f7e912617bd852c30766dc48edfeb74bb470dfd9/apps/site/next-data/generators/websiteFeeds.mjs#L31
• data - https://github.com/nodejs/nodejs.org/blob/f7e912617bd852c30766dc48edfeb74bb470dfd9/apps/site/site.json#L12
• feed.rss2() - https://github.com/nodejs/nodejs.org/blob/f7e912617bd852c30766dc48edfeb74bb470dfd9/apps/site/next-data/providers/websiteFeeds.ts#L10

Expected behavior
A clear and concise description of what you expected to happen.

Actual behavior

Example: https://nodejs.org/en/feed/blog.xml

<?xml version="1.0" encoding="utf-8"?>
<rss version="2.0">
    <channel>
        <title>Node.js Blog</title>
        …
        <description>undefined</description>
        <lastBuildDate>Mon, 18 Aug 2025 19:33:35 GMT</lastBuildDate>
        <docs>https://validator.w3.org/feed/docs/rss2.html</docs>
        <generator>https://github.com/jpmonette/feed</generator>
        <language>en</language>

Versions (please complete the following information):

• feed: 5.1.0