RDF conflicts: Warning: Illegal offset type in isset or empty in path_metatags_html_head_alter() line 50

How to reproduce:
- Enable RDF and Path Metatags.

        foreach ($element['#attributes'] as $group => $value) {
          if (!empty($build_meta[$value]) && $build_meta[$value] == $group) {
            unset($head_elements[$name]);
          }
        }

Here $value can be array, so we get warning when execute empty($build_meta[$value]) code.