Failed to init FileSystemAssetSigner if 'ASSET_STORE_SECRET' is empty

[Steven-Chan]

I am using the same set of environment variable for skygear-server and py-skygear. While I had no problem using file system as asset store on skygear-server with ASSET_STORE_SECRET is set to empty, FileSystemAssetSigner throws exception when init.

I think it is the relevant code (in py-skygear/skygear/asset/fs.py):

        secret = options.asset_store_secret
        if not secret:
            raise Exception('Missing signing secret for fs asset store')

While there is no such checking in NewFileStore of skygear-server/pkg/server/asset/fs.go (https://github.com/SkygearIO/skygear-server/blob/master/pkg/server/asset/fs.go).