In the expression builder, there are some functions listed.  one of the is IFNULL.  Use it like this:  QuantityOnHand / IFNULL(UDF_PackageQty,1).  (Of course, use the real field names for QOH and your UDF).  If the UDF field is null, it will replace it with a 1.  If the QOH is zero, the calculation will be 0/1 = 0.  

{fn IFNULL("IM_ItemWarehouse"."QuantityOnHand", 1)}/{fn IFNULL("CI_Item"."UDF_PACK_QTY", 1)}

This still results in a "divide by zero" error.    I think the problem is that Explore sees zero as zero & not null.   Unless it has some sort of a "standard" IF statement that I am SOL...

{fn IFNULL("IM_ItemWarehouse"."QuantityOnHand", 1)}/{fn IFNULL("CI_Item"."UDF_PACK_QTY", 1)}

This still results in a "divide by zero" error.    I think the problem is that Explore sees zero as zero & not null.   Unless it has some sort of a "standard" IF statement that I am SOL...

It is not Null which would be a blank field, it has a value, zero. So you should say something like if QtyOnHand is not = 0.00 then divide this by that otherwise 0.00