Increase unit test coverage for policy parser.

Change-Id: If801873d95c0157e3a243924b2da96a119e2acb0
diff --git a/patrole_tempest_plugin/rbac_policy_parser.py b/patrole_tempest_plugin/rbac_policy_parser.py
index 38bed7c..69a9842 100644
--- a/patrole_tempest_plugin/rbac_policy_parser.py
+++ b/patrole_tempest_plugin/rbac_policy_parser.py
@@ -101,7 +101,7 @@
             try:
                 file_policy_data = json.loads(file_policy_data)
             except ValueError:
-                pass
+                file_policy_data = None
 
         # Check whether policy actions are defined in code. Nova and Keystone,
         # for example, define their default policy actions in code.