Empty list [] to return all validators balances (#7474)

The endpoint `/eth/v1/beacon/states/head/validator_balances` returns an empty data when the data field is `[]`. According to the beacon API spec, it should return the balances of all validators:

Reference: https://ethereum.github.io/beacon-APIs/#/Beacon/postStateValidatorBalances
`If the supplied list is empty (i.e. the body is []) or no body is supplied then balances will be returned for all validators.`


  This PR changes so that: `curl -X 'POST' 'http://localhost:5052/eth/v1/beacon/states/head/validator_balances' -d '[]' | jq` returns balances of all validators.

beacon_node/http_api/src/lib.rs

@@ -709,7 +709,7 @@ pub fn serve<T: BeaconChainTypes>(
         .clone()
         .and(warp::path("validator_balances"))
         .and(warp::path::end())
-        .and(warp_utils::json::json())
+        .and(warp_utils::json::json_no_body())
         .then(
             |state_id: StateId,
              task_spawner: TaskSpawner<T::EthSpec>,