[BUG] Don't panic on empty blockquote

- On a empty blockquote the callout feature would panic, as it expects
to always have at least one child.
- This panic cannot result in a DoS, because any panic that happens
while rendering any markdown input will be recovered gracefully.
- Adds a simple condition to avoid this panic.

modules/markup/markdown/callout/github.go

@@ -36,6 +36,10 @@ func (g *GitHubCalloutTransformer) Transform(node *ast.Document, reader text.Rea
 
 		switch v := n.(type) {
 		case *ast.Blockquote:
+			if v.ChildCount() == 0 {
+				return ast.WalkContinue, nil
+			}
+
 			// We only want attention blockquotes when the AST looks like:
 			// Text: "["
 			// Text: "!TYPE"