In which case could the parameter be null? We only receive JSON from the dump files, and there we will always have a page title. I would simply ignore the redundant information in JSON. As you say, it is not clear what it means if it is contradicting. The page title (from the dump) would overrule the information inside the JSON (since MediaWiki uses the page title in all page-level operations and won't read the JSON unless it for displaying the page). So one has the option to either throw an error or to silently ignore the contradiction and use the page title. If this would ever happen, Wikibase would probably do the latter as well, so I think it is safe if we are not strict here.