Return nil instead or an error if no metrics

The docker metricset returns an error when there are no memory metric
values available.

This condition can happen when there are no running containers on
Docker.

When no containers are running, the metricset returns an error
at every collection, creating noise.

metricbeat/module/docker/memory/memory.go

@@ -72,7 +72,9 @@ func (m *MetricSet) Fetch(r mb.ReporterV2) error {
 
 	memoryStats := m.memoryService.getMemoryStatsList(stats, m.dedot)
 	if len(memoryStats) == 0 {
-		return fmt.Errorf("No memory stats data available")
+		// No memory stats available, probably because
+		// no containers are running.
+		return nil
 	}
 	eventsMapping(r, memoryStats)