[9.x] Skip adding search constraints with empty search on DatabaseEngine

[rodrigopedra]

Right now if we search for an empty string using the DatabaseEngine, for example in a endpoint that lists all users:

Route::get('/api/users', fn () => User::search(request()->query('q'))->paginate());

Even when the search string is empty, the where clauses are added:

select *
from `users`
where (`users`.`name` like '%%' or `users`.`email` like '%%' or match (`users`.`notes`) against ('' in natural language mode))
order by `id` desc
limit 30 offset 0

This PR skips adding the search where clauses with an empty search string.