fix(api): allow empty Lua table for nested dicts #22268

Problem:
The Lua-API bridge allows Dict params to be empty Lua (list) tables at
the function-signature level. But not for _nested_ Dicts, because they
are not modeled:
fae7540732/src/nvim/api/keysets.lua (L184)
Some API functions like nvim_cmd check for kObjectTypeDictionary and
don't handle the case of empty Lua tables (treated as "Array").

Solution:
Introduce VALIDATE_T_DICT and use it in places where
kObjectTypeDictionary was being checked directly.

fixes #21005

src/nvim/highlight.c

@@ -1025,7 +1025,7 @@ HlAttrs dict2hlattrs(Dict(highlight) *dict, bool use_rgb, int *link_id, Error *e
     // TODO(clason): handle via gen_api_dispatch
     cterm_mask_provided = true;
   } else if (HAS_KEY(dict->cterm)) {
-    VALIDATE_T("cterm", kObjectTypeDictionary, dict->cterm.type, {
+    VALIDATE_EXP(false, "cterm", "Dict", api_typename(dict->cterm.type), {
       return hlattrs;
     });
   }