[#952] Access log_format key correctly even if it's None
[#952] Access log_format key correctly even if it's None

This is the case of the infamous 'Unhandled error in Deferred' events
that were not logging the traceback.

pixelated/pixelated-user-agent,pixelated-project/pixelated-user-agent,pixelated/pixelated-user-agent,pixelated/pixelated-user-agent,pixelated/pixelated-user-agent,pixelated-project/pixelated-user-agent,pixelated-project/pixelated-user-agent,pixelated-project/pixelated-user-agent,pixelated-project/pixelated-user-agent,pixelated/pixelated-user-agent