RemovalClient handles null authenticator

Test: atest FaceREmovalClientTest
Test: called onRemoved({}) in FaceHAL and system_server doesn't crash
Bug: 219081263

This change fixes an issue which was presented by an AIDL workaround.
There is currently no API to indicate removeEnrollments() failed. So
onRemoved({}) is called instead. The framework does not correctly
handle this case, this change adds the necessary logic.

Change-Id: Iaf907f7afc00e5b897ac73947d0b5d5576621bdf