Add null check to contextual_tooltip ClearPrefs

Bug: 1058598
Change-Id: I37c233c9fd0aca75be049a2e9ce21498b7ecb7c9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2088671
Commit-Queue: Yulun Wu <yulunwu@chromium.org>
Reviewed-by: Manu Cornet <manucornet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#747356}

ash/shelf/contextual_tooltip.cc

@@ -137,7 +137,8 @@ void ClearPrefs() {
       Shell::Get()->session_controller()->GetLastActiveUserPrefService(),
       prefs::kContextualTooltips);
   base::DictionaryValue* nudges_dict = update.Get();
-  nudges_dict->Clear();
+  if (nudges_dict && !nudges_dict->empty())
+    nudges_dict->Clear();
 }
 
 void OverrideClockForTesting(base::Clock* test_clock) {