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

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)]]))