Fix database_name() method to return null instead of "???"

app/Models/StandaloneMongodb.php

@@ -224,6 +224,6 @@ class StandaloneMongodb extends BaseModel
         return $this->morphMany(ScheduledDatabaseBackup::class, 'database');
     }
     public function database_name() {
-        return "???";
+        return null;
     }
 }