Fix username lookup returning "null" instead of null (#3220)

common/src/main/java/me/lucko/luckperms/common/storage/implementation/mongodb/MongoStorage.java

@@ -573,7 +573,10 @@ public class MongoStorage implements StorageImplementation {
         MongoCollection<Document> c = this.database.getCollection(this.prefix + "uuid");
         Document doc = c.find(new Document("_id", uniqueId)).first();
         if (doc != null) {
-            return doc.get("name", String.class);
+            String username = doc.get("name", String.class);
+            if (username != null && !username.equals("null")) {
+                return username;
+            }
         }
         return null;
     }

common/src/main/java/me/lucko/luckperms/common/storage/implementation/sql/SqlStorage.java

@@ -694,7 +694,10 @@ public class SqlStorage implements StorageImplementation {
                 ps.setString(1, uniqueId.toString());
                 try (ResultSet rs = ps.executeQuery()) {
                     if (rs.next()) {
-                        return rs.getString("username");
+                        String username = rs.getString("username");
+                        if (username != null && !username.equals("null")) {
+                            return username;
+                        }
                     }
                 }
             }