OutgoingEmail#getUserNameEmailFor: Protect against null accountId

If the accountId is null, just return null.

Change-Id: Ibb6994a5b137fc898e54eef11d676fec35a1fad7

gerrit-server/src/main/java/com/google/gerrit/server/mail/send/OutgoingEmail.java

@@ -378,6 +378,9 @@ public abstract class OutgoingEmail {
    * @return name/email of account, username, or null if unset.
    */
   public String getUserNameEmailFor(Account.Id accountId) {
+    if (accountId == null) {
+      return null;
+    }
     AccountState who = args.accountCache.get(accountId);
     String name = who.getAccount().getFullName();
     String email = who.getAccount().getPreferredEmail();