Author's roles being `null` errors badly in Lua

From @roaldarbol in https://github.com/quarto-dev/quarto-cli/discussions/11055#discussioncomment-10934773_

---
title: "Untitled"
format: html
author:
  - name: Some name
    roles: ~
---

Content

This will throw uncatch Lua error

Error running filter C:/Users/chris/DOCUME~1/DEV_R/QUARTO~2/src/resources/filters/main.lua:
...sources\pandoc\datadir\..\..\filters\modules/authors.lua:747: bad argument #1 to 'for iterator' (table expected, got string)
stack traceback:
	...sources\pandoc\datadir\..\..\filters\modules/authors.lua:747: in upvalue 'parseRole'
	...sources\pandoc\datadir\..\..\filters\modules/authors.lua:779: in upvalue 'processAuthorRoles'
	...sources\pandoc\datadir\..\..\filters\modules/authors.lua:1001: in upvalue 'processAuthor'
	...sources\pandoc\datadir\..\..\filters\modules/authors.lua:1079: in function 'modules/authors.processAuthorMeta'
	...QUARTO~2/src/resources/filters/./normalize/normalize.lua:36: in function <...QUARTO~2/src/resources/filters/./normalize/normalize.lua:34>
	[C]: in ?
	[C]: in method 'walk'
	...V_R/QUARTO~2/src/resources/filters/./ast/customnodes.lua:76: in function <...V_R/QUARTO~2/src/resources/filters/./ast/customnodes.lua:65>
	(...tail calls...)
	...V_R/QUARTO~2/src/resources/filters/./ast/customnodes.lua:87: in function 'run_emulated_filter'
	..._R/QUARTO~2/src/resources/filters/./ast/runemulation.lua:82: in local 'callback'
	..._R/QUARTO~2/src/resources/filters/./ast/runemulation.lua:100: in upvalue 'run_emulated_filter_chain'
	..._R/QUARTO~2/src/resources/filters/./ast/runemulation.lua:136: in function <..._R/QUARTO~2/src/resources/filters/./ast/runemulation.lua:133>
stack traceback:
	...V_R/QUARTO~2/src/resources/filters/./ast/customnodes.lua:76: in function <...V_R/QUARTO~2/src/resources/filters/./ast/customnodes.lua:65>
	(...tail calls...)
	...V_R/QUARTO~2/src/resources/filters/./ast/customnodes.lua:87: in function 'run_emulated_filter'
	..._R/QUARTO~2/src/resources/filters/./ast/runemulation.lua:82: in local 'callback'
	..._R/QUARTO~2/src/resources/filters/./ast/runemulation.lua:100: in upvalue 'run_emulated_filter_chain'
	..._R/QUARTO~2/src/resources/filters/./ast/runemulation.lua:136: in function <..._R/QUARTO~2/src/resources/filters/./ast/runemulation.lua:133>

And YAML validation does not help here it seems.

We should probably handle null values for roles correctly, and throw more nicely