If some cells are undefined, the filter can generate an invalid AST

[wlupton]

If some cells are undefined, the filter can generate an invalid AST. For example, with this markdown:

::: list-table
* - A
  - B

* - 1
:::

...we get the following (edited to show just the offending rows; rep.lua reports the AST using the logging module; see below):

% pandoc-3.1.8 -L list-table.lua list-table-bug.md -L rep.lua -t html -o /dev/null
(#) blocks Blocks[1] {
  [1] Table {
    ...
    bodies: List[1] {
      [1] {
        ...
        body: List[1] {
          [1] pandoc.Row {
            ...
            cells: List[1] {
              [1] pandoc.Cell {
                alignment: "AlignDefault"
                attr: Attr {
                  attributes: AttributeList {}
                  classes: List {}
                  identifier: ""
                }
                col_span: 1
                contents: Blocks[1] {
                  [1] Plain {
                    content: Inlines[1] {
                      [1] Str "1"
                    }
                  }
                }
                row_span: 1
              }
            }
          }
        }
        ...

The row only contains a single cell, so this is an invalid AST and isn't handled correctly by all writers (see jgm/pandoc#9096 for further discussion).

The filter should be updated to add empty cells for any missing ones.

Here's rep.lua

local logging = require 'logging'
function Pandoc(pandoc)
    if logging.loglevel > 0 then
        logging.temp('meta', pandoc.meta)
    end
    logging.temp('blocks', pandoc.blocks)
end