Removing possible AV on nullptr in DSE preconnect

It is possible for the default search provider to be empty, and we
should avoid crashing when it is.

Bug: 1049861
Change-Id: I21ad12dfa7f6be5a582101a2081ae2725b3c3da8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2042055
Auto-Submit: Ryan Sturm <ryansturm@chromium.org>
Reviewed-by: Tarun Bansal <tbansal@chromium.org>
Commit-Queue: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#739265}

chrome/browser/navigation_predictor/search_engine_preconnector.cc

@@ -117,7 +117,8 @@ GURL SearchEnginePreconnector::GetDefaultSearchEngineOriginURL() const {
       Profile::FromBrowserContext(browser_context_));
   if (!template_service)
     return GURL();
-  return template_service->GetDefaultSearchProvider()
-      ->GenerateSearchURL({})
-      .GetOrigin();
+  const auto* search_provider = template_service->GetDefaultSearchProvider();
+  if (!search_provider)
+    return GURL();
+  return search_provider->GenerateSearchURL({}).GetOrigin();
 }