How to deal with Select Menu when it can NULL or a Value with SQL

Question

I have a SELECT Menu(:P_55X) with:
"NULL DISPLAY VALUE" = NULL
"NULL RETURN VALUE" = NULL
The Chart query is as follows.
SELECT NULL LINK,
START_DATE "FOR_MONTH",
ROUND(AVG("SLA"),3) "SLA_UPTIME"
from SLA_TIMES_ZABBIX
LEFT JOIN (SELECT DISTINCT NODE_NAME,OS_TYPE from CMS.CMS_NODE_OS where remove_dt is NULL) OSNODE on LOWER(SLA_TIMES_ZABBIX.NODE_NAME)=LOWER(OSNODE.NODE_NAME)
where OS_TYPE=:P55_X group by START_DATE order by START_DATE ASC;
How can I accout for the (:P55_X) variable being NULL sometimes and not null other times. meaning I can't use the "=" when the values is null because I need a IS NULL there.
The column in the database either has a value or is NULL.
Thanks
Robert

Answer 1

where (OS_TYPE = :P55_X OR (:P55_X IS NULL and OS_TYPE IS NULL))