By intention a search does not force the user to enter search parameters. If the user does not enter any parameters the system will display all available object instances (actually, only the first portion if the list is large - for practical reasons).
If you do want to enforce a search parameter, the only option at the moment would be to configure a non-persistent object with a mandatory attribute, let the user create the object, and then use the attribute value to search for actual objects. This needs to be wrapped in a process.
