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
diff --git a/tools/releasetools/test_ota_from_target_files.py b/tools/releasetools/test_ota_from_target_files.py
index b556b3a..8266908 100644
--- a/tools/releasetools/test_ota_from_target_files.py
+++ b/tools/releasetools/test_ota_from_target_files.py
@@ -33,7 +33,7 @@
     GetTargetFilesZipWithoutPostinstallConfig,
     Payload, PayloadSigner, POSTINSTALL_CONFIG,
     StreamingPropertyFiles, AB_PARTITIONS)
-from apex_utils import GetApexInfoFromTargetFiles
+from apex_utils import GetSystemApexInfoFromTargetFiles
 from test_utils import PropertyFilesTestCase
 
 
@@ -281,9 +281,9 @@
         metadata)
 
   @test_utils.SkipIfExternalToolsUnavailable()
-  def test_GetApexInfoFromTargetFiles(self):
+  def test_GetSystemApexInfoFromTargetFiles(self):
     target_files = construct_target_files(compressedApex=True)
-    apex_infos = GetApexInfoFromTargetFiles(target_files)
+    apex_infos = GetSystemApexInfoFromTargetFiles(target_files)
     self.assertEqual(len(apex_infos), 1)
     self.assertEqual(apex_infos[0].package_name, "com.android.apex.compressed")
     self.assertEqual(apex_infos[0].version, 1)