Skip "null" string when building advanced button summary

- Also added help string to SoundSettings as a side fix.

Change-Id: Ia81a46c6e199b72b52ae1fa9d996c094193b506c
Fix: 37933524
Test: robotests

src/com/android/settings/notification/SoundSettings.java

@@ -75,6 +75,11 @@ public class SoundSettings extends DashboardFragment {
         }
     }
 
+    @Override
+    protected int getHelpResource() {
+        return R.string.help_url_sound;
+    }
+
     @Override
     public void onPause() {
         super.onPause();