Require non-null HAT for fingerprint enrollment

Fingerprint service providers expect to be given a non-null HAT and will
crash otherwise. Documents that any HAT passed to these providers must be
non-null to avoid crashing system server. Also adds logging to catch a
potential source of a null HAT.

Test: Manual

Bug: 193811614
Change-Id: Ib05541653eab04390e9928ada37bde8d088cbc84