Fix cloudfront setup when no request policies exist

Setup fails when no cloudfront policies exist in an account. This
likely happens if the account is new or CloudFront hasn't been used
with it before.

cdn_proxy/cloudfront/main.py

@@ -319,7 +319,7 @@ def create_distribution(self, lambda_arn):
 
         resp = client.list_origin_request_policies(Type="custom")
 
-        for policy in resp["OriginRequestPolicyList"]["Items"]:
+        for policy in resp["OriginRequestPolicyList"].get("Items", []):
             if policy["OriginRequestPolicy"]["OriginRequestPolicyConfig"]["Name"] == policy_name:
                 policy_id = policy["OriginRequestPolicy"]["Id"]