Fix NPE due to a null package name of system enforcing admin.

mPackageName is assumed to be non-null. Set it to an empty string to
avoid a null pointer exception during saveToXml().

Bug: 358733540
Flag: EXEMPT bugfix
Test: - start MD emulator
      - Start New Profile
      - $ adb shell dumpsys user
      - $ adb shell dumpsys device_policy

Change-Id: I962e2bb89c573fdaaa29b41f5b6c81df881cb56c
diff --git a/services/devicepolicy/java/com/android/server/devicepolicy/EnforcingAdmin.java b/services/devicepolicy/java/com/android/server/devicepolicy/EnforcingAdmin.java
index e65e513..634f1bc 100644
--- a/services/devicepolicy/java/com/android/server/devicepolicy/EnforcingAdmin.java
+++ b/services/devicepolicy/java/com/android/server/devicepolicy/EnforcingAdmin.java
@@ -203,7 +203,8 @@
         // Only system authorities use this constructor.
         mIsSystemAuthority = true;
         mIsRoleAuthority = false;
-        mPackageName = null;
+        // Package name is not used for a system enforcing admin, so an empty string is fine.
+        mPackageName = "";
         mSystemEntity = systemEntity;
         mUserId = UserHandle.USER_SYSTEM;
         mComponentName = null;