refactor(gen_api_dispatch): avoid trailing spaces (#32632)

Problem:
Generated Lua API bindings may have trailing spaces (e.g. nvim_set_hl).

Solution:
Add leading spaces only if arg_free_code is non-empty.
Also:
- Remove an unnecessary string.format() in write_shifted_output() args.
- Fix incorrect indent for the nlua_push_keydict() below.

src/nvim/generators/gen_api_dispatch.lua

@@ -901,7 +901,7 @@ exit_0:
     local ret_mode = (ret_type == 'Object') and '&' or ''
     if fn.has_lua_imp then
       -- only push onto the Lua stack if we haven't already
-      write_shifted_output(string.format(
+      write_shifted_output(
         [[
     if (lua_gettop(lstate) == 0) {
       nlua_push_%s(lstate, %sret, kNluaPushSpecial | kNluaPushFreeRefs);
@@ -909,7 +909,7 @@ exit_0:
       ]],
         return_type,
         ret_mode
-      ))
+      )
     elseif string.match(ret_type, '^KeyDict_') then
       write_shifted_output(
         '    nlua_push_keydict(lstate, &ret, %s_table);\n',
@@ -927,7 +927,6 @@ exit_0:
 
     -- NOTE: we currently assume err_throw needs nothing from arena
     write_shifted_output(
-
       [[
   %s
   %s