Error from atom.xml when no pages have dates

[JVimes]

System information

If no pages have dates, I get:

Error: Failed to serve the site
Error: Failed to render 'atom.xml'
Error: Reason: Variable last_updated not found in context while rendering 'atom.xml'

Expected behaviour

No error. I'm not familiar enough with Atom to say what should be done, e.g. if it's trying to include a page it should not, or if it should leave out <updated> (which might be required?).

Actual behaviour

An error happens (above).

Steps to reproduce

1. Set up a new site with tabi
2. Set generate_feeds = true
3. Create just one page, _index.md, without a date variable.
4. Run zola serve.

Additional context

No response

Final checklist

• I've checked that the issue isn't already reported.
• I've tested with the latest version of tabi to check if the issue has already been fixed.