Fixed non-zero default policy index

Fixes integer conditional in the proxy container controller.
_backend_requests adds the policy header if the policy_index provided
evaluates to true. If policy with index 0 is not the default it can
never be set as it evalutes to false. Switched this statement to check
if policy_index is not None.

Closes-Bug: #1246107
Change-Id: I34a0551af1f07db76bb16844f6d11b515737a38f
Merge "Fixed non-zero default policy index" into feature/ec