Fix panic in cgroups monitoring (#21355)

Gosigar's cgroups GetStatsForProcesses can return a nil Stats pointer
and no error when the ["blkio", "cpu", "cpuacct", "memory"] subsystems
are on the root cgroup.

Related #21113

libbeat/cmd/instance/metrics.go

@@ -285,6 +285,10 @@ func reportBeatCgroups(_ monitoring.Mode, V monitoring.Visitor) {
 		logp.Err("error getting group status: %v", err)
 		return
 	}
+	// GetStatsForProcess returns a nil selfStats and no error when there's no stats
+	if selfStats == nil {
+		return
+	}
 
 	if cpu := selfStats.CPU; cpu != nil {
 		monitoring.ReportNamespace(V, "cpu", func() {