I remember that, in previous versions of the Search API and Search API Solr, by default not entering any search term would return the entire list of items.
I have tried with new empty Drupal 8.5.1 using Search API and Search API Solr, using the default configuration, with MariaDB and Postgresql, using the default view pages /search/content and /solr-search/content, but it shows an empty list and the message Please enter some keywords to search. message.
Does somebody know how to change it to the previous behavior to get again the whole list of elements when no search terms are used?