Use `/` as redirect location if webroot is set to an empty value

If the webroot has been set to an empty value or ownCloud has been installed at the root location (`/``) there is a fair chance that the redirect for password resets does not work at all.

This means that while the password is getting resetted the user is not redirected to the login/index page.

I'm aware that it might be better to just set the webroot to `/` in those cases but this patch is better in the regard that it cannot break stuff.

Thanks to @PVince81 for helping me debugging this. (I'm a moron and assumed it couldn't be THAT easy)

Reported by @cdamken

@karlitschek No-brainer in my opinion that we should backport. Blue one.