If the preferences doesn't have a url set, you probably have a default set in your global internet preferences. ISP's often set this when you sign up for service. You have several choices:
(1) Reset the homepage in your internet preferences. This is in one of the preferences panels, I forget which one.
(2) Type a new start page in the preferences for Safari. This is useful if you often start from a particular page.
(3) I believe there's a preference in Safari as to whether to start each new window with a blank url or your start page. I'll look to see how to set this when I get home. (Office computer is windows
-D