[ADI][21/N] fix NPE when stageDir is null

The stageDir can be null when the session is a parent session. Fix the
null pointer exception that occurs in that case.

FLAG: android.content.pm.verification_service
BUG: 360129657
Test: atest CtsRootDeveloperVerificationVerifierTestCases

Merged-In: Ib9d21cba93a5dda0e846d133f0998b558ef2dc3c
Change-Id: Ib9d21cba93a5dda0e846d133f0998b558ef2dc3c