Adopt latest changes to request hash computation

The `requestType` of empty lists is no longer part of the requests hash.

- ethereum/EIPs#8989

Author: etan-status

beacon_chain/spec/helpers.nim

@@ -462,7 +462,7 @@ func computeRequestsHash(
 
   let requestsHash = computeDigest:
     template mixInRequests(requestType, requestList): untyped =
-      block:
+      if requestList.len > 0:
         let hash = computeDigest:
           bind h
           h.update([requestType.byte])