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.
N/A
2 changes:
1. Replace Option::map_or(true, ...) with is_none_or(...)
2. Remove unnecessary `Into::into` blocks where the type conversion is apparent from the types
