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

test/functional/api/command_spec.lua

@@ -24,7 +24,7 @@ describe('nvim_get_commands', function()
     eq({}, meths.get_commands({builtin=false}))
   end)
 
-  it('validates input', function()
+  it('validation', function()
     eq('builtin=true not implemented', pcall_err(meths.get_commands,
       {builtin=true}))
     eq("Invalid key: 'foo'", pcall_err(meths.get_commands,