An example would be:
If filter_purchase = 0 Or \"\" Then SetDocVar \"filter_purchase\", \"0\" Else SetDocVar \"filter_purchase\", CStr(filter_purchase) End I
you have to explicitly state the condition for each OR. Please see below
If filter_purchase = 0 Or filter_purchase = "" Then SetDocVar "filter_purchase", "0" Else SetDocVar "filter_purchase", CStr(filter_purchase) End If