[bugfix] fn:transform: Conversion and treeIndex problems

[nverwer]

Description

This is a retry for #5680 which was botched after I changed my develop-6.x.x branch.

The fn:transform function sometimes produced NullPointerExceptions. This happened for documents containing nodes of type org.exist.dom.memtree.ElementImpl and other subtypes of org.exist.dom.memtree.NodeImpl.

The function org.exist.xquery.functions.fn.transform.Convert.ToSaxon.ofNode(Node) uses org.exist.xquery.functions.fn.transform.TreeUtils.treeIndex(Node) to get a 'path' (as a list of indexes) of the Node in its containing document. This 'path' is used to find the node in a newly built document that is suitable for use by Saxon.
Using the 'path' that the treeIndex function produces assumes that it starts at the document node.

However, some nodes do not have a containing document. Specifically, in org.exist.dom.memtree.NodeImpl.getParentNode():

if (parent.getNodeType() == DOCUMENT_NODE && !((DocumentImpl) parent).isExplicitlyCreated()) {
            /*
                All nodes in the MemTree will return an Owner document due to how the MemTree is implemented,
                however the explicitlyCreated flag tells us whether there "really" was a Document Node or not.
                See https://github.com/eXist-db/exist/issues/1463
             */
            return null;

In this case, the 'path' returned by treeIndex does not contain the index (0) for the root node, and org.exist.xquery.functions.fn.transform.TreeUtils.xdmNodeAtIndex(XdmNode, List<Integer>) will return null when a node is expected.

The proposed fix tests for this case. Going up the ancestor chain, when a node that is not a document node and that does not have a parent is found, the index 0 for this root element is inserted in the 'path'.

The PR also adds support for using maps and arrays as parameters in a XSLT transformation.

Testing

The NullPointerException happened in a rather complicated XQuery with documents composed from several sources.
I have not yet been able to make a simple test case.

[dizzzz]

I recognize that under some conditions there is not a "DocumentNode"; I need to recall the context, it is some time ago.

[reinhapa]

Just a small note...