Views Search API with Search API database, sort handler
In case you are using Search API Database Search as a backend for the Search API you can solve the problem by using hook_search_api_db_query_alter hook:

/**
 * Implements hook_search_api_db_query_alter().
 *
 * Preprocesses a search's database query before it is executed.
 *
 * @param SelectQueryInterface $db_query
 *   The database query to be executed for the search. Will have "item_id" and
 *   "score" columns in its result.
 * @param SearchApiQueryInterface $query
 *   The search query that is being executed.
 *
 * @see SearchApiDbService::preQuery()
 */
function custom_module_search_api_db_query_alter(SelectQueryInterface &amp;$db_query, SearchApiQueryInterface $query) {
  $db_query-&gt;orderby('ISNULL(field_field_name)', 'ASC');
}


This solved it and moved items with NULL values to bottom of the view. Cheers!