Content EXIM: add update only method in import task

Besides method

DocumentVariantImportTask#createOrUpdateDocumentFromVariantContentNode(ContentNode contentNode, String primaryTypeName, String documentLocation, String locale, String localizedName)

I'd like

DocumentVariantImportTask#updateDocumentFromVariantContentNode(ContentNode contentNode, Node documentNode)

for existing handles only (handleNode argument may not be null).

This is on behalf of a plugin that either has the handle already or creates it beforehand using DocumentManager#translateDocument.

Note that the documentLocation argument in the first method is not the exact same as a handle JCR path so using handle.getPath() as argument value sometimes fails, see also HIPFORGE-148.