Thanks for the PR, but I think this isn't a good idea. We don't to allow description: null without being pretty careful with the rest of the codebase. For example, we use description in Typescript:

https://github.com/quarto-dev/quarto-cli/blob/main/src/project/types/website/listing/website-listing-feed.ts#L123

I'm sure there are other places.

I concede that we should be documenting these and validating the YAML in listings. But I think description ought to always be a string.

We don't to allow description: null without being pretty careful with the rest of the codebase.

For a custom listing with a custom template is there any guarantee that you'll have any fields? I guess if so it'd be good to document the required fields.

Anyway, this change isn't really arguing that you should allow description to be something else; more I'm suggesting that this is a place where the undefined guard and Typescript's type checks don't fully cover the possible inputs. Checking item.description !== undefined just opens the door to a runtime error when isPlaceholder() makes the incorrect assumption that item.description has to be a string.

The error is super opaque, btw

ERROR: TypeError: Cannot read properties of null (reading 'match')

Stack trace:
    at isPlaceHolder (file:///Users/garrick/.local/share/qvm/versions/v1.7.23/bin/quarto.js:98176:17)
    at file:///Users/garrick/.local/share/qvm/versions/v1.7.23/bin/quarto.js:98846:48
    at Array.map (<anonymous>)
    at templateMarkdownHandler (file:///Users/garrick/.local/share/qvm/versions/v1.7.23/bin/quarto.js:98807:33)
    at markdownHandler (file:///Users/garrick/.local/share/qvm/versions/v1.7.23/bin/quarto.js:99832:24)
    at file:///Users/garrick/.local/share/qvm/versions/v1.7.23/bin/quarto.js:99734:31
    at Array.forEach (<anonymous>)
    at listingHtmlDependencies (file:///Users/garrick/.local/share/qvm/versions/v1.7.23/bin/quarto.js:99733:24)
    at eventLoopTick (ext:core/01_core.js:175:7)
    at async Object.formatExtras (file:///Users/garrick/.local/share/qvm/versions/v1.7.23/bin/quarto.js:100263:41)

It'd be better to validate item.description earlier or to have isPlaceholder() throw a more informative error, etc. But these two lines are equivalent in intent and behavior, except the second doesn't throw a runtime error:

if (item.description !== undefined && !isPlaceHolder(item.description)) {
if (typeof item.description === "string" && !isPlaceHolder(item.description)) {

I agree that we should be validating! But the validation should really be "item.description must be string", instead of "item.description might be null".