按 Null 字母顺序排序，然后按 Not Null 排序

问：

我有一个数据集，我尝试先按 null startdate 值排序，然后按姓氏排序。然后是非空开始日期，也按姓氏：

  declare @t table
  (lastname varchar(100),
  firstname varchar(50),
  startdate date)

  insert into @t
  values('Smith', 'John', '01/01/2023'),
  ('Marshall', 'Tom', NULL),
  ('Thomas', 'Frank', '01/01/2022'),
  ('Schmidt', 'Mike', NULL),
  ('Carter', 'Gary', '03/01/2023'),
  ('Belle', 'Albert', NULL)

  LastName     FirstName       StartDate
 Smith         John               2023-01-01
 Marshall      Tom               NULL
 Thomas        Frank             2022-01-01
 Schmidt       Mike              NULL
 Carter        Gary             2023-03-01
 Belle         Albert           NULL

期望输出：

  LastName     FirstName        StartDate
   Belle         Albert           NULL
   Marshall       Tom             NULL
   Schmidt        Mike            NULL
   Carter        Gary             2023-03-01
   Smith         John             2023-01-01
   Thomas        Frank            2022-01-01     

我试过做一个联盟：

  select * 
  into #nulls
  from @t
  where startdate is null
  order by lastname

  select * 
  into #nonnulls
  from @t
  where startdate is not null
  order by lastname

  select *
  from #nulls
  UNION
  select *
  from #nonnulls

我还尝试过使用按大小写排序：

  select *
  from @t
  order by case when startdate is null then lastname 
                when startdate is not null then lastname end