Update www_root empty check

[kyriog]

In nginx puppet manifest used in puphpet, a vhost can be generated without root instruction if the www_root directive isn't set (which, in my case, is useful because the root clause is declared by a including a file using include_files).

The problem: as this manifest check for '' and not undef, the check fails on the if ! defined(File[$vhost['www_root']]) test.

I think PuPHPet website should be updated too to remove www_root directive if the value is empty on configurator, but that's over my knowledge of this project.