OpenIdServiceImpl: Fix incorrect null comparison of Optional

AccountManager.lookup() returns an Optional, not a nullable value,
so it's incorrect to compare to null. Instead, check the value of
the isPresent() method.

This was detected by SonarLint.

Change-Id: Iec0d2beee9059ecd0b53a6815a9b1f8a9ac41c50

java/com/google/gerrit/httpd/auth/openid/OpenIdServiceImpl.java

@@ -192,7 +192,7 @@ class OpenIdServiceImpl {
     // We might already have this account on file. Look for it.
     //
     try {
-      return accountManager.lookup(aReq.getIdentity()) == null;
+      return !accountManager.lookup(aReq.getIdentity()).isPresent();
     } catch (AccountException e) {
       logger.atWarning().withCause(e).log("Cannot determine if user account exists");
       return true;