Quarto shortcode kwargs contain empty Pandoc Inlines for unprovided named parameters, not nil

Quarto shortcode kwargs contain empty Pandoc Inlines for unprovided named parameters, not nil

When writing a Quarto Lua shortcode extension, checking kwargs["key"] == nil to detect missing named arguments does not work. Quarto pre-populates the kwargs table with empty Pandoc Inlines objects ([]) for keys that were NOT passed in the shortcode invocation. This means kwargs["missing_key"] is not nil — it's an empty Inlines that pandoc.utils.stringify() converts to "".

This causes default values to silently fail:

-- BROKEN: default "Four" is never returned because kwargs["of"] is [] not nil
local function kwarg(kwargs, key, default)
  local v = kwargs[key]
  if v == nil then return default end
  return pandoc.utils.stringify(v)
end

-- Called as: {{< section part="One" >}}  (no of= provided)
local of = kwarg(kwargs, "of", "Four")  -- returns "" not "Four"

Symptoms: shortcode output has empty strings where defaults should appear; conditional branches like if kwarg_value then trigger on empty Inlines (truthy in Lua) when the parameter was never passed.