Check for null while comparing responsible installer & requested installer

If an archived app's installer is uninstalled, there is no "responsible
installer" for that app. While resintalling such an app, treat it like a
normal install instead of an unarchival

Test: Manual. Follow repro steps from the bug
Bug: 364804420
Flag: EXEMPT. Bug fix only
Change-Id: I26085e015531d0e63d2f13cd3d4d4c33c257727f