When OC_URL_SIGNING_SECRET is not set. Fall back to the value of the
reva transfer token. This allows handling upgrades on a instance that
was created before the OC_URL_SIGNING_SECRET was introduced to be
handled more graceful.
Unfortunately this still only works reliably for single instance
deployments (or instance that where bootstrapped using 'opencloud init')
that are guaranteed to have the transfer token available.
When running 'proxy' and 'ocdav' as separate services the upgrade might
still require manual intervention.
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