Mark credential as NonNull for successful response.

GetCredentialResponse will only be returned when the operation succeeds,
meaning it returns a valid credential. Therefore, mark the credential
property as NonNull.

Bug: 246564035
CTS-Coverage-Bug: 246637346
Test: Local Build & Deployment
Change-Id: I7464f29d47f552fa0ff269f00c7910abd8e551c0