Fix NPE of ToggleAccessibilityServicePreferenceFragmentTest.

ComponentName.unflattenFromString() return null if the string was
incorrect.

Bug: 218975511
Test: make RunSettingsRoboTests ROBOTEST_FILTER=ToggleAccessibilityServicePreferenceFragmentTest
Change-Id: I4384aa60aca657b5dd2ffc3a6fa44b7258bbb5b6

tests/robotests/src/com/android/settings/accessibility/ToggleAccessibilityServicePreferenceFragmentTest.java

@@ -83,9 +83,8 @@ public class ToggleAccessibilityServicePreferenceFragmentTest {
     }
 
     @Test
-    public void getTileName_noTileServiceAssigned_noMatchString() {
-        final CharSequence tileName = mFragment.getTileName();
-        assertThat(tileName.toString()).isEqualTo("");
+    public void getTileName_noTileServiceAssigned_returnNull() {
+        assertThat(mFragment.getTileName()).isNull();
     }
 
     @Test