Fix test case fail

The implementation in PreferenceGroup is changed,
contentEquals doesn't accept null value.

We filter null key case in InstrumentedPreferenceFragment.

Test: robotest
Change-Id: I575d96361edf9bf0b20f7ad4c61d8b3813bf31d9
Fixes: 121232342

src/com/android/settings/core/InstrumentedPreferenceFragment.java

@@ -22,6 +22,7 @@ import android.text.TextUtils;
 import android.util.Log;
 
 import androidx.annotation.XmlRes;
+import androidx.preference.Preference;
 import androidx.preference.PreferenceScreen;
 
 import com.android.settings.overlay.FeatureFactory;
@@ -78,6 +79,14 @@ public abstract class InstrumentedPreferenceFragment extends ObservablePreferenc
         updateActivityTitleWithScreenTitle(getPreferenceScreen());
     }
 
+    @Override
+    public <T extends Preference> T findPreference(CharSequence key) {
+        if (key == null) {
+            return null;
+        }
+        return super.findPreference(key);
+    }
+
     protected final Context getPrefContext() {
         return getPreferenceManager().getContext();
     }