am: Don't crash if cacheDir can't be created

 * Due to a bug in the multiuser code, it's possible for us to reach
   here by loading framework-res for a different user in which case the
   cache dir can't be created.
 * Add a null check to avoid the crash and subsequent lockup of AM.

Change-Id: I12f3a207d9fa0111286a67e29652f808ad5d9410