private void validateOptional(SchemaValidator validator, String file) throws IOException {
        if ( ! appDirs.file(file).exists()) return;
        validate(validator, file);
    }
Validation of xml files in application package against RELAX NG schemas.

@author hmusum
config-application-package/src/main/java/com/yahoo/config/model/application/provider/ApplicationPackageXmlFilesValidator.java