Oracle SQL: Fill missing values (gaps) with previous value

create table tq84_tab (
   nm  number,
   tx  varchar2(5)
);

insert into tq84_tab values ( 1,  null);
insert into tq84_tab values ( 2, 'abc');
insert into tq84_tab values ( 3,  null);
insert into tq84_tab values ( 4,  null);
insert into tq84_tab values ( 5, 'def');
insert into tq84_tab values ( 6,  null);
insert into tq84_tab values ( 7,  null);
insert into tq84_tab values ( 8,  null);
insert into tq84_tab values ( 9, 'ghi');
insert into tq84_tab values (10,  null);

select
   nm,
   tx,
   nvl (                   -- If current value of tx
      tx,                  -- is not null, then evaluate to tx,
      lag(tx)              -- otherwise, evaluate to preceding
        ignore nulls       -- non-null value of tx.
        over (order by nm) -- order by clause required to
                           -- determine «preceding»
   ) as tx_
from
   tq84_tab;
--
--    NM TX    TX_ 
-- ----- ----- -----
--     1           
--     2 abc   abc 
--     3       abc 
--     4       abc 
--     5 def   def 
--     6       def 
--     7       def 
--     8       def 
--     9 ghi   ghi 
--    10       ghi 

drop   table tq84_tab;