Fix registrar security console

The registrar security console failed because it assumed the email is a
required field for the registrar, but it isn't (at least - create_registrar
doesn't require an email, and update_registrar lets you remove the email).

Fixed by allowing it to *remain* unset if it was unset originally, but if it was set - it's required.

There are more fixes needed, but they aren't related to the email, so they will wait for the next CL

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=191623034

javatests/google/registry/server/RegistryTestServer.java

@@ -88,6 +88,8 @@ public final class RegistryTestServer {
       route("/registrar", google.registry.module.frontend.FrontendServlet.class),
       route("/registrar-settings",
           google.registry.module.frontend.FrontendServlet.class),
+      route("/registrar-premium-price-ack",
+          google.registry.module.frontend.FrontendServlet.class),
       route("/registrar-payment",
           google.registry.module.frontend.FrontendServlet.class),
       route("/registrar-payment-setup",