HierarchyPath parser now returns nil when
the path is empty (instead of a string slice with one
EmptyString element).
If the prefix is set to true, when calling the AsString
method on a nil HierarchyPath, only the separator will
be returned. This avoids a nil expr error coming from
the xmlquery library.
Use the Query and QueryAll functions from the xmlquery
package to be able to handle the errors ourselves and
avoid panics.
Remove config default value for xml_root_path. The field
will remain commented in config_defaults for reference.
Add tests for HierarchyPath.AsString function.
Add comments for XmlProvider and xml_root_path opt.
