Avoid entering an infinite loop in PackageManagerBackupAgent#onRestore

When the PackageManagerBackupAgent restores an old backup that does not
have an ancestral record version and the backup contains a package that
does not have any signatures it will enter an infinite loop. This
happens because it calls `continue` without reading the next key/value
pair in the backup.

Test: atest PackageManagerBackupAgentTest
Bug: 330708733
Change-Id: Ifb71293b75ccb68dc7e721593f04128417360491