Maybe related to https://gerrit.wikimedia.org/r/#/c/225287/ ("Get rid of default=wikipedia assumptions in config")/T104088, https://en.wikipedia.org/w/api.php?action=sitematrix&smsiteprop=dbname|url&smlangprop=site returns url set to null for Wikipedia sites, while it should be for example https://en.wikipedia.org for dbname = "enwiki".