Return nil when mw.entity.label and sitelink don't find anything

this is the old behavior and we ought not to change this, else
we break various lua modules like on Russian Wikipedia:

https://ru.wikipedia.org/wiki/%D0%A8%D0%B0%D0%B1%D0%BB%D0%BE%D0%BD:Wikidata

Bug: 73615
Change-Id: I57b825e9020d3e6910188edcbe2da728fa2bef54