Make Router reject classes with no @Action-returning methods

This provides a safeguard against using TypeInstantiator to resolve the component class, where if resolution is done incorrectly, you end up with java.lang.Object.  Formerly, that would have "succeeded" in generating a Router for Object, which of course has no methods that return @Action classes.  Such a router is pretty useless, so it's better to make Router stricter and have it fail if you give it such a class by accident.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=148353224

javatests/google/registry/request/RequestHandlerTest.java

@@ -363,6 +363,7 @@ public final class RequestHandlerTest {
   @Test
   public void testNullness() {
     NullPointerTester tester = new NullPointerTester();
+    tester.setDefault(Class.class, Component.class);
     tester.setDefault(RequestAuthenticator.class, requestAuthenticator);
     tester.setDefault(XsrfTokenManager.class, xsrfTokenManager);
     tester.testAllPublicStaticMethods(RequestHandler.class);