Lag ignore nulls alternative, PostgreSQL

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create table tbl (empid int,type varchar(10),tmestamp varchar(100),block_id varchar(100));
insert into tbl values 
(1     ,  'R' , '2018-04-15 01:13:15','AB12D'),
(1     ,  'P' , '2018-04-15 05:13:15',null),
(1     ,  'P' , '2018-04-15 05:13:15',null),
(1     ,  'P' , '2018-04-15 05:13:15',null),
(1     ,  'D' , '2018-04-15 07:13:15',null),
(1     ,  'D' , '2018-04-15 08:13:15',null),
(1     ,  'D' , '2018-04-15 10:13:15',null),
(1     ,  'R' , '2018-04-15 13:13:00','1X1#1'),
(1     ,  'P' , '2018-04-15 13:15:15',null),
(1     ,  'P' , '2018-04-15 13:15:15',null),
(1     ,  'P' , '2018-04-15 13:15:15',null),
(1     ,  'D' , '2018-04-15 14:13:00',null),
(1     ,  'D' , '2018-04-15 15:13:00',null),
(1     ,  'D' , '2018-04-15 16:13:37',null),
(2     ,  'R' , '2018-04-15 04:15:00','__08XA'),
(2     ,  'P' , '2018-04-15 04:20:00',null),
(2     ,  'D' , '2018-04-15 05:11:33',null)
;

select empid,type,tmestamp,max(block_id) over(partition by empid,grp order by tmestamp) as block_id
from (select t.*
      ,sum(case when block_id is null then 0 else 1 end) over(partition by empid order by tmestamp) as grp
      from tbl t
     ) t


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