Fix default icon

Credential entries with or without icons will always have non-null icon reference.
The valid icons will have non-null drawable reference but invalid
ones will have null drawable reference. If the drawable reference is null, then
use the default icon instead of the icon reference that is stored in the
map which won't display anything.

Bug: 299321128
Test: local device testing

Change-Id: Ib20046f5a8e3c66fb546bf6e8dde49388160739e