Assert the search returned results (.search-results), or — with "no" or "not" — that it reported none, via the module's default "Your search yielded no results." message.
Examples
Then I should see search results
And we should see search results
Then should see search results
And I should see no search results
Then we should not see search results
Pattern
Then(/^(I |we )*should( not)* see (no )?search results$/, …)
Part of @webship-js/drupal-core-steps (github.com/webship/drupal-core-steps), a step pack for Webship-js 2.0.x.