Small speed up for url()

I noticed that although there is a static cache for the $script variable in url() (which holds either ‘index.php’ or ” depending on the used web server), it is not implemented properly. url() checks if the variable is empty. On Apache, this is always true and the check for other server version is run every time the function is called. Instead, it should check for isset() to only run the check once.

This can also be backported to 5 and 4.7.