If no input is specified and there are validation errors, then an error is thrown.
Here is the rekevant code from the RequestProcessor:
String input = mapping.getInput();
if (input == null) {
if (log.isTraceEnabled()) {
log.trace(" Validation failed but no input form available");
}
response.sendError(HttpServletResponse.SC_INTERNAL_SERVER_ERROR,
getInternal().getMessage("noInput",
mapping.getPath()));
return (false);
}