[monodoc] Fix ecma traversal with unattached tree.

[monodoc] Fix ecma traversal with unattached tree.

Previously the code expected to stop at upper level node using the fact that their element starts with `root:/'. This element is attributed by RootNode when loading a complete tree. However tool like mdoc also loads tree unattached which doesn't create a standard `root:/` top-level root node hence causing the issue.