Mark nullable parameters as nullable

A few nullable parameters were not marked as nullable, which causes exceptions
to be thrown in debug mode.

This had no effect in the deployed web server, because these assert sanity
checks aren't performed - but on our local test server this failed.

Note that all these fields are checked for "nullness" in the code itself. It's
just an oversight in the declaration.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=226187227

java/google/registry/ui/soy/registrar/SecuritySettings.soy

@@ -18,7 +18,7 @@
 /** Registrar security settings page for view and edit. */
 {template .settings}
   {@param ipAddressWhitelist: list<string>}
-  {@param phonePasscode: string}
+  {@param? phonePasscode: string}
   {@param? clientCertificate: string}
   {@param? clientCertificateHash: string}
   {@param? failoverClientCertificate: string}

java/google/registry/ui/soy/registrar/WhoisSettings.soy

@@ -19,8 +19,8 @@
 /** Registrar whois settings page for view and edit. */
 {template .settings}
   {@param clientIdentifier: string}
-  {@param ianaIdentifier: int}
-  {@param icannReferralEmail: string}
+  {@param? ianaIdentifier: int}
+  {@param? icannReferralEmail: string}
   {@param readonly: bool}
   {@param? whoisServer: string}
   {@param? url: string}