Explicit null-check before use in FileIntegrityService

Null-ness check are moved to the beginning where the data comes in.

Also remove an unused AssertError which was originally to prevent
crashing system server in unexpected case.

Test: work-in-progress GTS test passed
Bug: 142573505
Change-Id: I7f7afa43dc60a8131cf937273d51781a1033efe0