The following is oracle code:
IF V_JDYWDM='LHCKGL' THEN
...
...
ELSEIF V_JDYWDM='XSCKGL' THEN
NULL;
ELSEIF V_JDYWDM='YWYKGL' THEN
NULL;
ELSEIF V_JDYWDM='YHGL' THEN
SELECT MAX(SPDM00) INTO SPDM FROM
(SELECT SPDM00,SUM(SL0000) FROM
(SELECT SPDM00,DECODE(SIGN(SL0000),-1,0,SL0000) SL0000 FROM LSTYWDQZBKC0_JK1 WHERE YWGSDM=V_LYGSDM
AND SPDM00 IN (SELECT SPDM00 FROM XSTYHFP2 WHERE GSDW00=V_LYGSDM AND DHAO00=V_LYDHAO AND SL0000<0)
UNION ALL SELECT SPDM00,SL0000 FROM XSTYHFP2 WHERE GSDW00=V_LYGSDM AND DHAO00=V_LYDHAO AND SL0000<0)
GROUP BY SPDM00 HAVING SUM(SL0000)<0);
IF SPDM IS NOT NULL THEN
SELECT MAX(SPMC00) INTO SPMC FROM WMVSPXX0 WHERE SPDM00=SPDM;
FLAG:=1;
TEXT:=V_LYGSDM||'-'||V_LYGSMC||'-'||SPDM||'-'||SPMC;
RETURN;
END IF;
END IF;

IS there like null statement in DB2? null mean do nothing

try

then
set xyz=null

i think that should work

the null in oracle language as statement means no operation (not the value null)
I don't see any statement that could do the same
just not coding the condition would do, but this implies code change..