Check for null in getDeviceOwnerOrProfileOwnerPackage

This prevents a system crash when running SessionTest on secondary
users.

bug: 138118013
bug: 138120084
Test: cts-tradefed run cts-dev -m CtsPackageInstallTestCases -t android.packageinstaller.install.cts.SessionTest
and cts-tradefed run cts-dev -m CtsPermission2TestCases -t android.permission2.cts.RestrictedPermissionsTest#onSideLoadRestrictedPermissionsWhitelistingDefault
Change-Id: I2be4d3121666c107aaf32113f7c13a9b5b3ff5dd