beacon/engine: check for empty requests (#31010)

According to
https://github.com/ethereum/execution-apis/blob/main/src/engine/prague.md#engine_newpayloadv4:

> Elements of the list MUST be ordered by request_type in ascending
order. Elements with empty request_data MUST be excluded from the list.

---------

Co-authored-by: lightclient <14004106+lightclient@users.noreply.github.com>

eth/catalyst/api.go

@@ -638,6 +638,9 @@ func (api *ConsensusAPI) NewPayloadV4(params engine.ExecutableData, versionedHas
 		return engine.PayloadStatusV1{Status: engine.INVALID}, engine.UnsupportedFork.With(errors.New("newPayloadV4 must only be called for prague payloads"))
 	}
 	requests := convertRequests(executionRequests)
+	if err := validateRequests(requests); err != nil {
+		return engine.PayloadStatusV1{Status: engine.INVALID}, engine.InvalidParams.With(err)
+	}
 	return api.newPayload(params, versionedHashes, beaconRoot, requests, false)
 }
 
@@ -727,6 +730,9 @@ func (api *ConsensusAPI) NewPayloadWithWitnessV4(params engine.ExecutableData, v
 		return engine.PayloadStatusV1{Status: engine.INVALID}, engine.UnsupportedFork.With(errors.New("newPayloadWithWitnessV4 must only be called for prague payloads"))
 	}
 	requests := convertRequests(executionRequests)
+	if err := validateRequests(requests); err != nil {
+		return engine.PayloadStatusV1{Status: engine.INVALID}, engine.InvalidParams.With(err)
+	}
 	return api.newPayload(params, versionedHashes, beaconRoot, requests, true)
 }
 
@@ -1287,3 +1293,20 @@ func convertRequests(hex []hexutil.Bytes) [][]byte {
 	}
 	return req
 }
+
+// validateRequests checks that requests are ordered by their type and are not empty.
+func validateRequests(requests [][]byte) error {
+	var last byte
+	for _, req := range requests {
+		// No empty requests.
+		if len(req) < 2 {
+			return fmt.Errorf("empty request: %v", req)
+		}
+		// Check that requests are ordered by their type.
+		if req[0] < last {
+			return fmt.Errorf("invalid request order: %v", req)
+		}
+		last = req[0]
+	}
+	return nil
+}