If the DocumentRoot value wasn't set, applying the state would cause the
following error:

          ID: site.example.com-documentroot
    Function: file.directory
        Name: None
      Result: False
     Comment: Specified file None is not an absolute path
     Started: 15:07:05.596723
    Duration: 4.929 ms
     Changes:

This fixes the problem by not trying to check for the DocumentRoot directory is
the value is missing.