Libs: Miscellaneous fixes (#3993)

* Fixed sceCompanionUtilGetEvent stub

Previously we effectively stubbed with ORBIS_COMPANION_UTIL_INVALID_POINTER, which makes no sense and caused issues in games.

* Check for null issuer_id in libSceNpAuth's GetAuthorizationCode

Comes up in Mirror's Edge Catalyst, according to some debugging done by a community member.
Given the library didn't have any null checks for that value, this is probably allowed.