fix home reference for setups where relative_base_url would be empty

[GertSallaerts]

This happens when your grav root folder is the same as your webroot, i.e. no subfolder sites.

Don't know if using base_url_absolute is the best way, but it works, I've also seen antimatter use {{ base_url == '' ? '/' : base_url }}

[hexplor]

Yeah, that might be a good idea. Thanks :)