Fix GFM callout rendering crash with empty content (#13603)

The GFM callout renderer would crash with internal_error() when
rendering callouts that had a title but no body content, because
it didn't handle the case where callout.content is nil.

This fix applies the same nil-safe pattern used by other format
renderers (EPUB, RevealJS): default to empty Blocks when content
is nil.

Author: cderv

news/changelog-1.9.md

@@ -17,6 +17,7 @@ All changes included in 1.9:
 ### `gfm`
 
 - ([#13421](https://github.com/quarto-dev/quarto-cli/issues/13421)): Do not word-wrap titles in header.
+- ([#13603](https://github.com/quarto-dev/quarto-cli/issues/13603)): Fix callouts with title but no body content causing fatal error when rendering to GitHub Flavored Markdown.
 
 ### `html`
 

src/resources/filters/customnodes/callout.lua

@@ -207,11 +207,12 @@ function _callout_main()
     if tt ~= "nil" then 
       result:insert(pandoc.Header(3, quarto.utils.as_inlines(callout.title)))
     end
-    local ct = pandoc.utils.type(callout.content)
+    local content = callout.content or pandoc.Blocks({})
+    local ct = pandoc.utils.type(content)
     if ct == "Block" then
-      result:insert(callout.content)
+      result:insert(content)
     elseif ct == "Blocks" then
-      result:extend(callout.content)
+      result:extend(content)
     else
       internal_error()
     end

tests/docs/smoke-all/2025/01/23/issue-13603.qmd

@@ -0,0 +1,43 @@
+---
+title: "Callout with title but no body (#13603)"
+format: gfm
+_quarto:
+  tests:
+    gfm:
+      ensureFileRegexMatches:
+        -
+          # Should render both callouts with proper GFM structure
+          - '\[!TIP\]'
+          - '\[!NOTE\]'
+          # Both should be in blockquotes with titles
+          - '>\s*###\s*Title Only'
+          - '>\s*###\s*With Body'
+          # The second callout should have content
+          - '>\s*Content here'
+        - []
+      noErrors: true
+---
+
+## Test callout with title but no body
+
+This tests the fix for issue #13603: callouts with a title but no body content
+should render to GitHub Flavored Markdown without crashing.
+
+The issue occurred because the GFM renderer didn't handle the case where
+`callout.content` is nil/empty.
+
+::: {.callout-tip}
+
+## Title Only
+
+:::
+
+## Callout with title and body (for comparison)
+
+::: {.callout-note}
+
+## With Body
+
+Content here.
+
+:::