Ignore null action in VcnManagementService$VcnBroadcastReceiver

Bug: 376183723
Test: Manual.
Change-Id: I079421b080942eb49db0c2aeb09e2c75378a0c3b
diff --git a/services/core/java/com/android/server/VcnManagementService.java b/services/core/java/com/android/server/VcnManagementService.java
index 51c768b..4a94edb 100644
--- a/services/core/java/com/android/server/VcnManagementService.java
+++ b/services/core/java/com/android/server/VcnManagementService.java
@@ -524,6 +524,9 @@
         @Override
         public void onReceive(Context context, Intent intent) {
             final String action = intent.getAction();
+            if (action == null) {
+                return;
+            }
 
             switch (action) {
                 case Intent.ACTION_PACKAGE_ADDED: // Fallthrough