I don't agree that a blank page will always be shown; at best I think this is container-dependent (since the explicit behaviour isn't given in the spec.). The default behaviour for HttpServlet in Tomcat and Sun AppServer (the Reference Implementation) is to return the HTTP status code 405 ("Method Not Allowed") which indicates to the client that there is no implementation for that HTTP method type. This is not the same as throwing an exception, and not the same as delegating to the superclass (GenericServlet). Whether the server decides to return a default error page or a blank page is container-dependent; you can even choose to return your own custom 405 error page!
I'm dubious about the integrity of the question: certainly the code will compile fine because there are default implementations for each method (returning a 405 status code), but I don't agree (unless other conditions were stipulated) that a blank page is guaranteed.
If you really want to be current, take a look at the source code for HttpServlet in Glassfish (the new Java EE 5 reference implementation):