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