nil metric list panic fix

CHANGELOG.md

@@ -36,6 +36,7 @@ should now look like:
 
 ### Bugfixes
 
+- [#1426](https://github.com/influxdata/telegraf/pull/1426): nil metrics panic fix.
 - [#1384](https://github.com/influxdata/telegraf/pull/1384): Fix datarace in apache input plugin.
 - [#1399](https://github.com/influxdata/telegraf/issues/1399): Add `read_repairs` statistics to riak plugin.
 - [#1405](https://github.com/influxdata/telegraf/issues/1405): Fix memory/connection leak in prometheus input plugin.

internal/models/running_output.go

@@ -138,7 +138,7 @@ func (ro *RunningOutput) Write() error {
 }
 
 func (ro *RunningOutput) write(metrics []telegraf.Metric) error {
-	if len(metrics) == 0 {
+	if metrics == nil || len(metrics) == 0 {
 		return nil
 	}
 	start := time.Now()

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, "."))
 	}