A secret_env entry with optional: true is skipped when its secret file is
missing, unreadable, or empty, instead of failing the whole resolution and
taking the app down. Needed for per-node integrations like btcpay's
internal-LND connection string: nodes without LND must still run btcpay.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>