releasetools: Skip on empty care_map.

common.GetCareMap() may return an empty list on unavailable care_map
since the change in commit 8bdfb990ea978e0cf4e16fc0702dc7cb067856e8.
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
diff --git a/tools/releasetools/add_img_to_target_files.py b/tools/releasetools/add_img_to_target_files.py
index 4af10ca..99ffa31 100755
--- a/tools/releasetools/add_img_to_target_files.py
+++ b/tools/releasetools/add_img_to_target_files.py
@@ -113,7 +113,7 @@
 
   Returns:
     (which, care_map_ranges): care_map_ranges is the raw string of the care_map
-    RangeSet; or an empty list.
+    RangeSet; or None.
   """
   assert which in common.PARTITIONS_WITH_CARE_MAP
 
@@ -123,7 +123,7 @@
   # invalid reads.
   image_size = OPTIONS.info_dict.get(which + "_image_size")
   if not image_size:
-    return []
+    return None
 
   image_blocks = int(image_size) / 4096 - 1
   assert image_blocks > 0, "blocks for {} must be positive".format(which)
@@ -592,7 +592,11 @@
         OPTIONS.info_dict.get(avb_hashtree_enable) == "true"):
       image_path = image_paths[partition]
       assert os.path.exists(image_path)
-      care_map_list += GetCareMap(partition, image_path)
+
+      care_map = GetCareMap(partition, image_path)
+      if not care_map:
+        continue
+      care_map_list += care_map
 
       # adds fingerprint field to the care_map
       build_props = OPTIONS.info_dict.get(partition + ".build.prop", {})