Fix 'cmd notification' crashes when no command is set.

Test: adb shell cmd notification
Fixes: 63705462

Change-Id: Icdeed5b67913c4bde525171b36f8d6103aa5dfbd

services/core/java/com/android/server/notification/NotificationManagerService.java

@@ -5746,6 +5746,9 @@ public class NotificationManagerService extends SystemService {
 
         @Override
         public int onCommand(String cmd) {
+            if (cmd == null) {
+                return handleDefaultCommands(cmd);
+            }
             final PrintWriter pw = getOutPrintWriter();
             try {
                 switch (cmd) {