let markdownToInlines take empty string as argument. (#6290)

* markdownToInlines: return empty list on empty string

* changelog

news/changelog-1.4.md

@@ -121,6 +121,7 @@
 - Add support `quarto.doc.add_resource` and `quarto.doc.add_supporting`. `add_resource` will add a resource file to the current render, copying that file to the same relative location in the output directory. `add_supporting` will add a supporting file to the current render, moving that file file to the same relative location in the output directory.
 - ([#6211](https://github.com/quarto-dev/quarto-cli/pull/6211)): Improve error message when a JSON filter (or a potentially misspelled Lua filter from an extension) is not found.
 - ([#6215](https://github.com/quarto-dev/quarto-cli/issues/6215)): Add `quarto.utils.string_to_inlines` and `quarto.utils.string_to_blocks` to Lua API to convert a string to a list of inlines or blocks taking into account quarto's AST structure.
+- ([#6289](https://github.com/quarto-dev/quarto-cli/issues/6289)): allow `markdownToInlines` to take empty string.
 
 ## Books
 

src/resources/filters/common/pandoc.lua

@@ -87,7 +87,11 @@ end
 function markdownToInlines(str)
   if str then
     local doc = pandoc.read(str)
-    return doc.blocks[1].content
+    if #doc.blocks == 0 then
+      return pandoc.List({})
+    else
+      return doc.blocks[1].content
+    end
   else
     return pandoc.List()
   end

tests/docs/smoke-all/2023/07/20/issue-6289.qmd

@@ -0,0 +1,12 @@
+---
+title: "Untitled"
+format: html
+---
+
+This is @tbl-t1.
+
+```{r}
+#| label: tbl-t1
+#| tbl-cap: ''
+gt::gt(mtcars)
+```