1. Create snmp item with non existing oid (to get the error "No Such Instance currently exists at this OID")

2. Create prepsocessing step Check for not supported value

2.1 set error matches is "No Such Instance"

2.2 set Custom on fail is <empty>

3. Test item

Result: Instead of an empty string the error will be duplicated as a value

 

If i chande "error matches" to "any error" - set empty string is working