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

java/google/registry/ui/externs/json.js

@@ -77,7 +77,7 @@ registry.json.Response.prototype.results;
  *   ianaIdentifier: (number?|undefined),
  *   icannReferralEmail: string,
  *   ipAddressWhitelist: !Array<string>,
- *   emailAddress: string,
+ *   emailAddress: (string?|undefined),
  *   phonePasscode: (string?|undefined),
  *   phoneNumber: (string?|undefined),
  *   faxNumber: (string?|undefined),