Ignore package sessions with empty package names.

This changelist prevents package sessions with empty package names from
being processed. Previously, the SessionInfo was directly translated
into a PackageInstallSession model. Since the package name field is
non-null, this can lead to a crash. The conversion now returns a
nullable result which is checked by the callers.

Fixes: 398478006
Flag: EXEMPT bugfix
Test: atest PackageInstallerMonitorTest#onCreateUpdatedSession_ignoreNullPackageNameSessions
Change-Id: I42703ec1f9664094aa0db70ed0c9911f10ade4d2