Updated HTTP auth check

Well... somehow I never read that second sentence of the [empty()
spec](http://us1.php.net/empty) description that says it doesn't produce
any warnings if the variable you pass doesn't exist, so yes, isset()
would be redundant, for sure. And yes, an initially empty false value is
a little easier.

So, it assumes there's no such authorization unless
`$_SERVER['REMOTE_USER']` isn't empty or it can find the "Authorization"
Apache request header. If HTTP authorization is being achieved by way of
passing values to `header()`, `$_SERVER['REMOTE_USER']` would need to be
set in the authentication process (such as with the value of
`$_SERVER['PHP_AUTH_USER']`) if they would like to use this utility.
What are your thoughts?