Validate tier manager is initialized in tier Empty() check (#14646)

Co-authored-by: Poorna Krishnamoorthy <poorna@minio.io>

cmd/tier.go

@@ -148,6 +148,9 @@ func (config *TierConfigMgr) Verify(ctx context.Context, tier string) error {
 
 // Empty returns if tier targets are empty
 func (config *TierConfigMgr) Empty() bool {
+	if config == nil {
+		return true
+	}
 	return len(config.ListTiers()) == 0
 }