How can I differentiate between 0 and a blank cell in Numbers?

I've been trying to figure this out for a while, but cant quite seem to get it

I need a formula which has the following outcome:

If the reference cell is blank, I need the formula cell to show 0 (If A1 is blank, B2 is 0), and if the reference cell is 0, i need the formula cell to show -5 (If A1 is 0, B2 is -5)

My problem is that Numbers keeps treating 0 and a blank cell as the same thing

I'm using Numbers 3.5.3

Any help would be appreciated