Description of problem:
System page does not render if the access policy contains no rules. 

Version-Release number of selected component (if applicable):
2571c4b (unreleased develop)

How reproducible:
easily

Steps to Reproduce:
1. Find or create a system which you own, with no rules in the access policy
(Note that by default there is a rule granting everybody "view", so you would need to remove that from a freshly created system.)
2. Open the system page

Actual results:
Page partially displays, but is not functional due to JS error.

Expected results:
Normal page behaviour, no JS errors. Access policy widget should appear empty, ready for rules to be added.

Additional info:
"TypeError: rules is null" in SystemAccessPolicyView#render.