Tom Fitch gave you an answer yesterday:
Put it in an If step, or in Set Variable and then you'd put the variable in your If step. E.g.,
Set Variable[ $emptyFieldsDetected ; <the aforementioned Case calculation> ]
If[ $emptyFieldsDetected ]
do stuff here
My question would be why does it matter if the source field has a value at all?
if field1 = "test" then Set Field field2 = field1 results in "test" in field1 and field2..
If field1="" then Set Field field2 = field1 results in "" in field1 and field2..
if you want to force something in field2 if field1 is empty then consider...
If field1 = "" then...
Set Field field2 = If ( IsEmpty ( field1 ) ; "empty" ; field1 ) results in "" in field1 and "empty" in field2