AccountValidator: Handle missing user branches

AccountConfig returns null from getAccount() if the user branch is
missing. Handle the null case in AccountValidator although this should
never happen (since all callers of AccountValidator get the account ID
from an existing user branch).

Change-Id: If781c8e15c5fa90a0f2d2bed3443c0285cc398f0
Signed-off-by: Edwin Kempin <ekempin@google.com>

java/com/google/gerrit/server/git/validators/AccountValidator.java

@@ -62,6 +62,10 @@ public class AccountValidator {
               newId.name(), AccountConfig.ACCOUNT_CONFIG, accountId.get(), e.getMessage()));
     }
 
+    if (newAccount == null) {
+      return ImmutableList.of(String.format("account '%s' does not exist", accountId.get()));
+    }
+
     List<String> messages = new ArrayList<>();
     if (accountId.equals(self.get().getAccountId()) && !newAccount.isActive()) {
       messages.add("cannot deactivate own account");
@@ -81,6 +85,7 @@ public class AccountValidator {
     return ImmutableList.copyOf(messages);
   }
 
+  @Nullable
   private Account loadAccount(Account.Id accountId, RevWalk rw, ObjectId commit)
       throws IOException, ConfigInvalidException {
     rw.reset();