Refresh Doctype Cache if it is empty

To make the anchestor searches more robust, we should refresh the doctype cache as soon as we find out that it is empty.

The problems encountered if the cache has been cleared, but not refreshed are nasty to track to their root cause. The downstream search will simply have their Ancestor property invalid. The upstream search may be cured with a simple doctype cache refresh.

The change will usually remove the need for a cache refresh for the upstream search. The downsteam search problem should also never be found again since the cache is refreshed on any problem encountered.