Log empty string for packages without logging_parent

Previously, if a package did not have a logging parent
then we would use the package itself for logging purposes.
Instead, use the empty string and a version code of 0 when
logging this case.

Test: atest WatchdogRollbackLoggerTest
Bug: 148669063
Change-Id: Ie2f86d3fc4c2ef17d344fc1dd5b3e70393db6a66