fix: tiering statistics handling a bug in clone() implementation (#18342)

Tiering statistics have been broken for some time now, a regression
was introduced in 6f2406b0b6

Bonus fixes an issue where the objects are not assumed to be
of the 'STANDARD' storage-class for the objects that have
not yet tiered, this should be conditional based on the object's
metadata not a default assumption.

This PR also does some cleanup in terms of implementation,

fixes #18070

cmd/tier.go

@@ -177,8 +177,24 @@ func (config *TierConfigMgr) Empty() bool {
 	return len(config.ListTiers()) == 0
 }
 
+// TierType returns the type of tier
+func (config *TierConfigMgr) TierType(name string) string {
+	config.RLock()
+	defer config.RUnlock()
+
+	cfg, ok := config.Tiers[name]
+	if !ok {
+		return "internal"
+	}
+	return cfg.Type.String()
+}
+
 // ListTiers lists remote tiers configured in this deployment.
 func (config *TierConfigMgr) ListTiers() []madmin.TierConfig {
+	if config == nil {
+		return nil
+	}
+
 	config.RLock()
 	defer config.RUnlock()