Remove logging and nullable return type from getCustomAction

There is no need to check boundary conditions and log potential errors
if no index is passed in. Nulls are not added to the resulting list,
so there's no reason for the nullable return type.

Bug: 224749799
Test: Cleanup only, so ran MediaDataManagerTest
Change-Id: Ie1d7eb8ce72a62a94da838e5f5589ef4c46cad8c