NULL is Ignored in Rule

Hi Experts.

I have written a below code and its working fine except one case. When the columns has null values in the SQL view, the IS is not checking up such values in the failed results. Though its violating the rule.

Here's my code, may be its wrong or it needs corrected.

BEGIN

IF ($ProcurementMethod = 'PO/SO'  And ($Source_Costing_SPK != $Source_MRPController))

RETURN FALSE;

ELSE IF ($ProcurementMethod = 'STO' OR $ProcurementMethod = 'STSA' AND ($Costing_SPK != ' '))

RETURN FALSE;                               

ELSE IF ($ProcurementMethod = 'STO' OR $ProcurementMethod = 'STSA' AND ($Source_Costing_SPK !=$Source_MRP_SPK))

RETURN FALSE;

ELSE

RETURN TRUE;

END

Appreciate your thoughts.

thanks in advance,

Vinay