nil metric list panic fix

plugins/serializers/graphite/graphite.go

@@ -26,6 +26,9 @@ func (s *GraphiteSerializer) Serialize(metric telegraf.Metric) ([]string, error)
 	timestamp := metric.UnixNano() / 1000000000
 
 	bucket := s.SerializeBucketName(metric.Name(), metric.Tags())
+	if bucket == "" {
+		return out, nil
+	}
 
 	for fieldName, value := range metric.Fields() {
 		// Convert value to string
@@ -89,6 +92,10 @@ func (s *GraphiteSerializer) SerializeBucketName(
 		}
 	}
 
+	if len(out) == 0 {
+		return ""
+	}
+
 	if s.Prefix == "" {
 		return sanitizedChars.Replace(strings.Join(out, "."))
 	}