fix(aurweb): resolve exception in ratelimit

Redis's get() method can return None which makes an RPC request error
out:

  File "/srv/http/aurweb/aurweb/ratelimit.py", line 103, in check_ratelimit
    requests = int(requests.decode())
AttributeError: 'NoneType' object has no attribute 'decode'
2 jobs for redis_cache in 2 minutes and 43 seconds (queued for 2 seconds)
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