Skip serializing proposer boost if null
## Issue Addressed
Restore compatibility between Lighthouse v2.0.1 VC and `unstable` BN in preparation for the next release.
## Proposed Changes
* Don't serialize the `PROPOSER_SCORE_BOOST` as `null` because it breaks the `extra_fields: HashMap<String, String>` used by the v2.0.1 VC.
@@ -659,6 +659,7 @@ pub struct Config {
#[serde(with = "eth2_serde_utils::quoted_u64")]
churn_limit_quotient: u64,
+ #[serde(skip_serializing_if = "Option::is_none")]
proposer_score_boost: Option<MaybeQuoted<u64>>,
#[serde(with = "eth2_serde_utils::quoted_u64")]
