Returns empty apex_infos if the apex target dir does not exist.

This is needed for partial builds that do not have the apex directory in
their target files package.

Test: build target files for a partial vendor-only build.
Change-Id: I076bfbd1a81cccddcef795f5edeaf2b51538cdec

tools/releasetools/apex_utils.py

@@ -516,7 +516,7 @@ def SignApex(avbtool, apex_data, payload_key, container_key, container_pw,
     raise ApexInfoError(
         'Failed to get type for {}:\n{}'.format(apex_file, e))
 
-def GetApexInfoFromTargetFiles(input_file):
+def GetSystemApexInfoFromTargetFiles(input_file):
   """
   Get information about system APEX stored in the input_file zip
 
@@ -538,6 +538,11 @@ def GetApexInfoFromTargetFiles(input_file):
     tmp_dir = UnzipTemp(input_file, ["SYSTEM/apex/*"])
   target_dir = os.path.join(tmp_dir, "SYSTEM/apex/")
 
+  # Partial target-files packages for vendor-only builds may not contain
+  # a system apex directory.
+  if not os.path.exists(target_dir):
+    return []
+
   apex_infos = []
 
   debugfs_path = "debugfs"