Previously the runner unconditionally invoked the passthrough DEK
resolver, which required a 32-byte key. Jobs configured with
encryption_enabled=false arrive with EncryptedDek=nil and the resolver
returned an 'expected 32-byte DEK, got 0' error, failing every run.
When EncryptedDek is empty the runner now skips the encrypt stage and
io.Copy()s the compressed stream straight into the upload pipe. The
encrypted_dek on BackupCompleted stays empty as well, matching the
server's expectation for an un-encrypted run.