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.

common/warp_utils/src/json.rs

@@ -31,3 +31,27 @@ pub fn json<T: DeserializeOwned + Send>() -> impl Filter<Extract = (T,), Error =
                 .map_err(|err| reject::custom_deserialize_error(format!("{:?}", err)))
         })
 }
+
+// Add a json_no_body function to handle the case when no body is provided in the HTTP request
+pub fn json_no_body<T: DeserializeOwned + Default + Send>(
+) -> impl Filter<Extract = (T,), Error = Rejection> + Copy {
+    warp::header::optional::<String>(CONTENT_TYPE_HEADER)
+        .and(warp::body::bytes())
+        .and_then(|header: Option<String>, bytes: Bytes| async move {
+            if let Some(header) = header {
+                if header == SSZ_CONTENT_TYPE_HEADER {
+                    return Err(reject::unsupported_media_type(
+                        "The request's content-type is not supported".to_string(),
+                    ));
+                }
+            }
+
+            // Handle the case when the HTTP request has no body, i.e., without the -d header
+            if bytes.is_empty() {
+                return Ok(T::default());
+            }
+
+            Json::decode(bytes)
+                .map_err(|err| reject::custom_deserialize_error(format!("{:?}", err)))
+        })
+}