Restore backwards compatibility when using older BNs (#3410)

## Issue Addressed

https://github.com/status-im/nimbus-eth2/issues/3930

## Proposed Changes

We can trivially support beacon nodes which do not provide the `is_optimistic` field by wrapping the field in an `Option`.

beacon_node/http_api/src/lib.rs

@@ -1704,7 +1704,7 @@ pub fn serve<T: BeaconChainTypes>(
 
                     let syncing_data = api_types::SyncingData {
                         is_syncing: network_globals.sync_state.read().is_syncing(),
-                        is_optimistic,
+                        is_optimistic: Some(is_optimistic),
                         head_slot,
                         sync_distance,
                     };