[Related Docs] SimilaritySearchRelatedDocsProvider buggy when scope is root node

When suggestions are requested for a document, a similarity search takes place that uses a node as the scope of the search. As described in the plugin's javadocs, this is the procedure to find the scope

// The search for related documents is within the scope of the 'root site content'
// find the root site node: We identify the root site node as follows: It is the first node from the jcr root up to
// the document documentModel.getNode() that has the mixin 'hippotranslation:translated' : If no such node with that
// mixin is found, we do the search relative to the jcr:root

In the case that the root node must be used (this is when no translated folders were found in the path of the document) then the search always fetches 0 results. That's because there is a condition added to the xpath query, it reads " and @hippo:paths='null' ". This condition must be removed when we have the root node as the scope of the search. Otherwise there are 0 results.