Guard HMGET against empty payload keys in GetWorkflowInstanceHistory

When an instance's history stream is empty, payloadKeys is empty and
HMGET is called with no fields, which Redis rejects with 'ERR wrong
number of arguments for hmget'. Guard the payload fetch with
len(payloadKeys) > 0, mirroring the existing guard in workflow.go.

Author: DerkSchooltink

backend/redis/instance.go

@@ -97,15 +97,17 @@ func (rb *redisBackend) GetWorkflowInstanceHistory(ctx context.Context, instance
 		events = append(events, event)
 	}
 
-	res, err := rb.rdb.HMGet(ctx, rb.keys.payloadKey(instance), payloadKeys...).Result()
-	if err != nil {
-		return nil, fmt.Errorf("reading payloads: %w", err)
-	}
-
-	for i, event := range events {
-		event.Attributes, err = history.DeserializeAttributes(event.Type, []byte(res[i].(string)))
+	if len(payloadKeys) > 0 {
+		res, err := rb.rdb.HMGet(ctx, rb.keys.payloadKey(instance), payloadKeys...).Result()
 		if err != nil {
-			return nil, fmt.Errorf("deserializing attributes for event %v: %w", event.Type, err)
+			return nil, fmt.Errorf("reading payloads: %w", err)
+		}
+
+		for i, event := range events {
+			event.Attributes, err = history.DeserializeAttributes(event.Type, []byte(res[i].(string)))
+			if err != nil {
+				return nil, fmt.Errorf("deserializing attributes for event %v: %w", event.Type, err)
+			}
 		}
 	}