I think this is a question of opinion. And maybe it was previously the case. But I think having many results is better to help the visitor... specially on small sites.
The only possibility would be to add a checkbox on service parameters to choose if empty search means all results or no results.
After 5 years of reflexions, I keep my opinion : launching an empty search that return everything is better.
I hate websites that answer : "enter at least one criteria"