fix: don't crash if fullname is not a string

https://community.nodebb.org/topic/15291/upgrade-error-fullname-substr-is-not-a-function

src/upgrades/1.15.0/fullname_search_set.js

@@ -16,7 +16,7 @@ module.exports = {
 			const userData = await user.getUsersFields(uids, ['uid', 'fullname']);
 			const bulkAdd = userData
 				.filter(u => u.uid && u.fullname)
-				.map(u => ['fullname:sorted', 0, u.fullname.substr(0, 255).toLowerCase() + ':' + u.uid]);
+				.map(u => ['fullname:sorted', 0, String(u.fullname).substr(0, 255).toLowerCase() + ':' + u.uid]);
 			await db.sortedSetAddBulk(bulkAdd);
 		}, {
 			batch: 500,