Fix NullPointerException thrown in jetty non-context (ServletHandler) servlet handling

[jianshaow]

If use ServletHandler as servlet handler, like below

ServletHandler servletHandler = new ServletHandler();
servletHandler.addServletWithMapping(TestServlet.class, "/hello");
jettyServer = new Server(0);
jettyServer.setHandler(servletHandler);

httpServletRequest.getContextPath() wil return null, add a null judgment can fix it