let me understand: you want an alert that triggers when there's a match between the two lookups, but you want that the alert triggers also when there's no match, is it correct?
if this is your need, you could try a different approach:
| inputlookup main.csv
| stats count BY eventSource
| append | inputlookup secondary.csv | eval count=0 | fields eventSource Name count
| stats values(name) AS name sum(count) AS total BY eventSource
| eval status=if(total>0, "match", "no match")
| table eventSource name status
in this way you have a table that lists all the situations (match, no match) that you can filter as you like and you'll have always a result.
One additional hint: in the lookup command, when you have the same fieldname in both lookups, you don't need to write "eventSource AS eventSource ", you can simply write the common key "eventSource " the notation "field1 AS field2" is for the situations when the field names are different.