self.redis isn't None, it's actually unset, so accessing it results
in an exception:
```
ERROR: Traceback (most recent call last):
File "./.venv/lib/python3.9/site-packages/starlette/routing.py", line 624, in lifespan
await receive()
File "./.venv/lib/python3.9/site-packages/starlette/routing.py", line 521, in __aexit__
await self._router.shutdown()
File "./.venv/lib/python3.9/site-packages/starlette/routing.py", line 608, in shutdown
await handler()
File "./etebase_fastapi/main.py", line 72, in on_shutdown
await redisw.close()
File "./etebase_fastapi/redis.py", line 18, in close
if self.redis is not None:
AttributeError: 'RedisWrapper' object has no attribute 'redis'
```
