fix virtual-root if script-name is ""

In d0cb841 (Avoid trailing slash in virtual-root), virtual-root was set
from script-name using trim_end().  However, if script-name was the
empty string (""), which happens when cgit is used to serve the root
path on a domain (/), trim_end() returns NULL and cgit acts like
virtual-root is not available.  Now, set virtual-root to "" in this
case, which fixes this bug.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>