query configuration for validate empty quote to zero

We just changed the postgresql server and got some problem with sql command
in our old server we can exexute this query:

SELECT * FROM product WHERE prdtnumber = ''

field prdtnumber data type is integer and the empty quote somehow can be assumed into 0 (zero)
Now, if I use the same query it return an error:
invalid input syntax for integer: ""

Anyone know what's the problem??

Thanks

Yudie