Fix crash introduced while cleaning up ui_proxy_config_service.cc

Crash was introduced here:
https://chromium-review.googlesource.com/c/chromium/src/+/1930832

Bug: 1047790
Change-Id: I4a603f4db03209e8e770b045ff981298988ccc74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2052201Reviewed-by: Andreea-Elena Costinas <acostinas@google.com>
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#741101}

chromeos/network/proxy/ui_proxy_config_service.cc

@@ -312,7 +312,7 @@ void UIProxyConfigService::OnPreferenceChanged(const std::string& pref_name) {
     return;
   const NetworkState* network =
       network_state_handler_->GetNetworkStateFromGuid(current_ui_network_guid_);
-  if (!network)
+  if (network)
     network_state_handler_->SendUpdateNotificationForNetwork(network->path());
 }