fix(api): allow empty list for cterm in nvim_set_hl

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`.)

1 files changed, 6 insertions, 0 deletions

diff --git a/test/functional/api/highlight_spec.lua b/test/functional/api/highlight_spec.lua
index 416945f379..03b407f4e0 100644
--- a/test/functional/api/highlight_spec.lua
+++ b/test/functional/api/highlight_spec.lua
@@ -242,6 +242,12 @@ describe("API: set highlight", function()
     eq(highlight2_result, meths.get_hl_by_name('Test_hl', false))
   end)
 
+  it ("can set emtpy cterm attr", function()
+    local ns = get_ns()
+    meths.set_hl(ns, 'Test_hl', { cterm = {} })
+    eq({}, meths.get_hl_by_name('Test_hl', false))
+  end)
+
   it ("cterm attr defaults to gui attr", function()
     local ns = get_ns()
     meths.set_hl(ns, 'Test_hl', highlight1)