Tendermint config optional values parsing

[tzemanovic]

The Tendermint config parser from toml uses a custom serde deserializing function for optional config values deserialize_optional_value, which requires an empty string when there's no value. If the optional field is missing from the config altogether, it fails with e.g.:

"missing field tls_cert_file for key rpc at line 96 column 1"

This could be relaxed as the Tendermint node does accept missing optional fields.

Furthermore, the serialization for the config omits the fields with optional values when there's no value, so it cannot parse back what it has written.