Fix typo that causes Assistant service failed to start

Bug: None
Test: Manual test
Change-Id: Ib88c098855428a0e96e66ba4fe18988afec05ebf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1717234Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#680599}

chromeos/services/assistant/service.cc

@@ -300,7 +300,7 @@ void Service::Init(mojom::ClientPtr client,
 void Service::OnPrefServiceConnected(
     std::unique_ptr<::PrefService> pref_service) {
   // TODO(b/110211045): Add testing support for Assistant prefs.
-  if (!pref_service_)
+  if (!pref_service)
     return;
 
   pref_service_ = std::move(pref_service);