Handle null NetworkState updates

Test: as follows
    - built (bullhead)
    - flashed
    - booted
    - runtest frameworks-net passes
    - manual USB tethering to WiFi and mobile switching works
Bug: 32163131
Change-Id: I2d4ef7c135ae6eb50cc31236f2fd20080536ea6a

services/core/java/com/android/server/connectivity/Tethering.java

@@ -1206,7 +1206,8 @@ public class Tethering extends BaseNetworkObserver implements IControlsTethering
 
             protected void handleNewUpstreamNetworkState(NetworkState ns) {
                 mIPv6TetheringCoordinator.updateUpstreamNetworkState(ns);
-                mOffloadController.setUpstreamLinkProperties(ns.linkProperties);
+                mOffloadController.setUpstreamLinkProperties(
+                        (ns != null) ? ns.linkProperties : null);
             }
         }