FolderArtifactHandler not respecting ImportMode on adjusting primaryTypes

By definition mode=merge or the new mode=merge_properties should never modify existing nodes/properties for the corresponding filters. It turns out that this check is missing for the adjustments of primaryNodeTypes which boldly breaks where the parent structures are assumed to be preexisting (like /home/groups) and just covered by folders without any nodetype declared - the existing logic does reset those to the default nt:folder which leads to a broken commit due to nodeType incompatibilities.