What does it mean when Spring MVC @Controller returns null view name?

I downloaded the code for the Spring MVC 3 Showcase. One thing puzzles me (well, more than one), why does this (edited for concision) sample return null?

@Controller
@RequestMapping("/form")
public class FormController {
    @RequestMapping(method=RequestMethod.POST)
    public String processSubmit(@Valid FormBean form, 
                BindingResult result, 
                WebRequest webRequest, 
                HttpSession session, Model model) {

        if (result.hasErrors()) {
             return null;
        } else {
            session.setAttribute("form", form);
            return "redirect:/form";            
        }
    }
}