Add check against empty source ID in references to controlled

vocabularies in wikidata fetcher

src/NodaWikidataFetcher.php

@@ -697,6 +697,7 @@ final class NodaWikidataFetcher {
 
         foreach ($nodaLinks as $noda_source => $nodaId) {
 
+            if ($nodaId === null) continue;
             if (!isset(self::URL_PREFIXES_PLACES_NODA_SOURCE[$noda_source])) {
                 throw new Exception("Unknown noda link: " . $noda_source);
             }