Problem: when accessing `nvim_set_hl` from Lua, empty tables are converted
to empty lists, not dictionaries, resulting in an error for
:lua vim.api.nvim_set_hl(0, "Comment", { cterm = {} })
Workaround: add an empty array as a special case when checking
`dict->cterm.type` and just set `cterm_mask_provided`.
(Proper solution: handle this in `gen_api_dispatch.lua`.)