Fix test_sign_apex

Test: python -m unittest test_sign_apex
Change-Id: I2a27a394b3de32fa29e487bea0b7721502a8bd98
diff --git a/tools/releasetools/apex_utils.py b/tools/releasetools/apex_utils.py
index dc6e3ca..6808f15 100644
--- a/tools/releasetools/apex_utils.py
+++ b/tools/releasetools/apex_utils.py
@@ -370,11 +370,12 @@
   extra_signapk_args = OPTIONS.extra_signapk_args[:]
   extra_signapk_args.extend(['-a', '4096'])
 
+  password = container_pw.get(container_key) if container_pw else None
   common.SignFile(
       aligned_apex,
       signed_apex,
       container_key,
-      container_pw.get(container_key),
+      password,
       codename_to_api_level_map=codename_to_api_level_map,
       extra_signapk_args=extra_signapk_args)