APEX

Hello all,
How does APEX check for null values in Text Fields on the forms? This might sound trivial but I have a problem with a PL/SQL Validation that I have written.
I have one select list (P108_CLUSTER_ID) and one Text field (P108_PRIVATE_IP). I made P108_CLUSTER_ID to return null value when nothing is selected and assumed P108_PRIVATE_IP to return null value too when nothign is entered in the text field.

All that I need is to validate if P108_PRIVATE_IP is entered when a P108_CLUSTER_ID is selected. i.e it is mandatory to enter Private IP when a cluster is seelcted and following is my Pl/SQL code

Declare
v_valid boolean;
Begin
IF :P108_CLUSTER_ID is NULL and :P108_PRIVATE_IP is NULL THEN
v_valid := TRUE;
ELSIF :P108_CLUSTER_ID is NOT NULL and :P108_PRIVATE_IP is NOT NULL THEN
v_valid := TRUE;
ELSIF :P108_CLUSTER_ID is NOT NULL and :P108_PRIVATE_IP is NULL THEN
v_valid := FALSE;
ELSIF :P108_CLUSTER_ID is NULL and :P108_PRIVATE_IP is NOT NULL THEN
v_valid := FALSE;
END IF;
return v_valid;
END;


My problem is it is returning FALSE for all the cases.It works fine in SQL Command though..When I tried to Debug and use Firebug, I found that Text fields are not stored a null by default but as empty strings "" . Now I tried modifying my PL/SQL to check Private_IP against an empty string. But doesn't help. Can someone please tell me how I need to proceed.

Thanks