Remove redundant empty string check in SCRAM normalization

The existing check for an empty normalized string is unnecessary because the OpaqueString profile in precis already throws an error if an empty string is returned: https://cs.opensource.google/go/x/text/+/master:secure/precis/profiles.go;l=66

smtp/auth_scram.go

@@ -310,8 +310,5 @@ func (a *scramAuth) normalizeString(s string) (string, error) {
 	if err != nil {
 		return "", fmt.Errorf("failed to normalize string: %w", err)
 	}
-	if s == "" {
-		return "", errors.New("normalized string is empty")
-	}
 	return s, nil
 }