Simplify certificate nil check in smime.go

Replaced redundant `nil` check with a concise length check for `keyPairTLS.Certificate`. This ensures cleaner and more readable code while maintaining the same functionality.

smime.go

@@ -126,7 +126,7 @@ func getLeafCertificate(keyPairTLS *tls.Certificate) (*x509.Certificate, error)
 		return keyPairTLS.Leaf, nil
 	}
 
-	if keyPairTLS.Certificate == nil || len(keyPairTLS.Certificate) == 0 {
+	if len(keyPairTLS.Certificate) == 0 {
 		return nil, errors.New("certificate chain is empty")
 	}
 	cert, err := x509.ParseCertificate(keyPairTLS.Certificate[0])