fix(libtofs): use select to deal with empty strings in path

* To prevent path breakage when any of the following settings are used
  in the pillar/config:

```
  tofs:
    path_prefix: ''
    dirs:
      files:     ''
      default:   ''
```

template/libtofs.jinja

@@ -87,12 +87,13 @@
         {%- else %}
           {%- set fs_dir = salt['config.get'](tplroot ~ ':tofs:dirs:default', 'default') %}
         {%- endif %}
-        {%- set url = '- salt://' ~ '/'.join([
+        {%- set url = [
+            '- salt:/',
             path_prefix_inc_ext,
             files_dir,
             fs_dir,
-            src_file.lstrip('/')
-        ]) %}
+            src_file,
+        ] | select | join('/') %}
 {{ url | indent(indent_width, true) }}
       {%- endfor %}
     {%- endfor %}