Styling of a search form is challenging at times. The default styling is very basic and an awful lot of the required styling depends on the CSS Reset used. The following is the styling I use on most of the WordPress themes I develop.
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.