Fix null in path names

[metabiswadeep]

Fix null in path names.

Related to PR 9 of pappl-retrofit.

[tillkamppeter]

Thanks for the pull request, but you can do it in a much simpler way. Simply add one line near the beginning of the function (~ line 3066)

if (root == NULL)
  root = "";

instead of all these ifs, especially also with all the { ... } when there is only a single statement.