Just recently I stumbled upon a website that offered a free accessibility report for a website. I thought this was just too good to be true but I entered my site and email details and waited a few minutes for the report to arrive. My first thoughts were correct, I should not have wasted my time.
If your WordPress theme originates from Underscores and you have not changed the default code in the footer there is a good chance that the Theme Check plugin will show that there is a possible hard-coded link in the code. This bit of information does not mean that there is anything wrong with your theme but for a user or prospective user of your theme, this bit of information could be problematic.
In WordPress you will see a list of all posts and pages on the site if you omit to add a search string and then click the submit button. The only reason that I can think of why WordPress does things this way is because the search functions are also used in the admin pages e.g. on All Pages and All Posts. On both these pages there are filters that include ‘All Categories’ and ‘All Dates’. For both these filters the search string will be empty but still WordPress has to return all the entries.