Remove NPE in onPackageFailure

It is possible for null to be returned by
ProcessRecord.getPackageListWithVersionCode on package failure. This
can cause a NPE in Package Watchdog. Ensure that the list of failing
packages is not null.

Test: atest PackageWatchdogTest
Bug: 151113966
Change-Id: Iab23cd6b4b8ae6b787df5f0b831b51e0ac8b3d31