Assert the search returned results, or, with "no" or "not", that it returned none. The results are the items of the list the Search module renders (ol.search-results; the "search results" named selector), or, where a theme drops that class, of the ordered list of the main region whose every item carries a linked heading. "No results" means the page ran a search and lists no item, so the assertion does not depend on the wording or the language of the "Your search yielded no results." text.
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-cms-steps (github.com/webship/drupal-cms-steps), a step pack for Webship-js 2.0.x.