Calculate two fields, hidden (blank) if one of the fields empty.

I have 3 fields
Field1 Field2 Field3
 
Field3 got custom calculation script:
 
var v1 = getField("Field1").value;
var v2 = getField("Field2").value;
event.value = v1 - v2;
event.target.hidden = (event.value <= 0);
 
When Field1 filled with number, Field3 got same number.
Need something to keep blank before Field2 filed with number.
 
Similar to Excel:
=IF(OR(Field1="",Field2=""),"",SUM(Field1-Field2))
 
Thanks in advance.
Igor.