fix(lua)!: do not use typed table for empty dict

Problem:
Empty dictionaries are converted into typed tables of the form `{ [true]
= 6}` instead of an empty dictionary representation `{}`. This leads to
incorrect table representation, along with failure in JSON encoding of
such tables as currently tables with only string and number type keys
can be encoded.

Solution:
The typed table logic has been removed from `nlua_push_Dictionary`. The
typed table logic is required only for float value conversions which is
already handled in `nlua_push_Float`. So, it is(was) no longer required
here.

Fixes neovim/neovim#29218

runtime/doc/news.txt

@@ -69,7 +69,8 @@ LSP
 
 LUA
 
-• TODO
+• API functions now consistently return an empty dictionary as
+  |vim.empty_dict()|. Earlier, a |lua-special-tbl| was sometimes used.
 
 OPTIONS
 

src/nvim/lua/converter.c

@@ -703,14 +703,10 @@ void nlua_push_Boolean(lua_State *lstate, const Boolean b, int flags)
 void nlua_push_Dictionary(lua_State *lstate, const Dictionary dict, int flags)
   FUNC_ATTR_NONNULL_ALL
 {
-  if (dict.size == 0 && (flags & kNluaPushSpecial)) {
-    nlua_create_typed_table(lstate, 0, 0, kObjectTypeDictionary);
-  } else {
-    lua_createtable(lstate, 0, (int)dict.size);
-    if (dict.size == 0 && !(flags & kNluaPushSpecial)) {
-      nlua_pushref(lstate, nlua_global_refs->empty_dict_ref);
-      lua_setmetatable(lstate, -2);
-    }
+  lua_createtable(lstate, 0, (int)dict.size);
+  if (dict.size == 0) {
+    nlua_pushref(lstate, nlua_global_refs->empty_dict_ref);
+    lua_setmetatable(lstate, -2);
   }
   for (size_t i = 0; i < dict.size; i++) {
     nlua_push_String(lstate, dict.items[i].key, flags);

test/functional/api/vim_spec.lua

@@ -1500,6 +1500,8 @@ describe('API', function()
       eq('Dictionary is locked', pcall_err(request, 'nvim_set_vvar', 'nosuchvar', 42))
       api.nvim_set_vvar('errmsg', 'set by API')
       eq('set by API', api.nvim_get_vvar('errmsg'))
+      api.nvim_set_vvar('completed_item', { word = 'a', user_data = vim.empty_dict() })
+      eq({}, api.nvim_get_vvar('completed_item')['user_data'])
       api.nvim_set_vvar('errmsg', 42)
       eq('42', eval('v:errmsg'))
       api.nvim_set_vvar('oldfiles', { 'one', 'two' })

test/functional/lua/vim_spec.lua

@@ -1205,8 +1205,7 @@ describe('lua stdlib', function()
     ]])
     eq(true, exec_lua([[return next(vim.fn.FooFunc(3)) == nil ]]))
     eq(3, eval('g:test'))
-    -- compat: nvim_call_function uses "special" value for empty dict
-    eq(true, exec_lua([[return next(vim.api.nvim_call_function("FooFunc", {5})) == true ]]))
+    eq(true, exec_lua([[return vim.tbl_isempty(vim.api.nvim_call_function("FooFunc", {5}))]]))
     eq(5, eval('g:test'))
 
     eq({ 2, 'foo', true }, exec_lua([[return vim.fn.VarArg(2, "foo", true)]]))