releasetools: Skip on empty care_map.

common.GetCareMap() may return an empty list on unavailable care_map
since the change in commit 8bdfb990.
Caller needs to handle such a case accordingly. This CL fixes the caller
in add_img_to_target_files.py, and changes the return value to None to
break legacy callers loudly.

Fixes: 131794385
Test: `atest releasetools_test`
Change-Id: I7c94f456064199237e84ef75732bdd10ebe31736