Make metered wifi controller resilient to null config

Apparently the wifi config can sometimes come back as null
initially when the device auto-connects to a network. This CL simply
makes the controller resilient to that so that it doesn't crash the
app.

Test: robotests
Bug: 73076869
Change-Id: I99098f35df9cd902f5f244a872cce9b71c17c224

tests/robotests/src/com/android/settings/wifi/details/WifiMeteredPreferenceControllerTest.java

@@ -83,4 +83,12 @@ public class WifiMeteredPreferenceControllerTest {
 
         assertThat(mDropDownPreference.getEntry()).isEqualTo("Detect automatically");
     }
+
+    @Test
+    public void testController_resilientToNullConfig() {
+        mPreferenceController = spy(new WifiMeteredPreferenceController(mContext, null));
+
+        mPreferenceController.getMeteredOverride();
+        mPreferenceController.onPreferenceChange(mDropDownPreference, new Integer(1));
+    }
 }