Fix to only init db that is specified in env(#30)

[kjyoun-ai]

Currently,

async onModuleInit() {
    try {
      console.log('Connecting to Redis...');
      this.client = await createClient({
        url: process.env.REDIS_URL,
        password: process.env.REDIS_PASSWORD,
      })
        .on('error', (err) => console.log('Redis Client Error', err))
        .connect()
      console.log('Connected to Redis successfully');
    } catch (e) {
      console.log('Failed to connect to Redis database', e)
    }
  }

is breaking the app by infinite connection trial to redis when Redis is not set up. It will block all other incoming API requests.
//todo this is rather quick fix and I would later refactor this code by lazy/dynamically load single db module depending on .env. So that app doesn't waste resource trying to load unnecessary module.